Chapter15  第15章

Optics is the branch of physics which deals with the behavior and properties of light, in, cluding its interactions with matter and the construction of instruments. In general, the term "light" in physics refers to electromagnetic radiation of any wavelength, whether visible of not. In this chapter, "light" is visible light that is visible to the human eye and electromagnetic radiation with a wavelength in a range from about to about , with a frequency range of about to .
光学是物理学的一个分支，它处理光的行为和特性，包括光与物质的相互作用和仪器的构造。一般来说，物理学中的“光”一词是指任何波长的电磁辐射，无论是否可见。在本章中，“光”是人眼可见的可见光和波长在大约 到大约 ，频率范围大约 到的 电磁辐射。

One of our principal contacts with the world around us is through light. We personally depend on light to convey visual information and have many instruments which use light to op. erate such as microscope, telescope and spectrometer. Most of what we know about the Universe comes from information that has been carried to us by light. Our knowledge on the structure of atoms comes largely from observing the radiations they emit. Therefore, light is very important to us, and nothing is visible to humans when light is totally absent.
我们与周围世界的主要联系之一是通过光。我们个人依靠光来传达视觉信息，并有许多使用光来操作的仪器，例如显微镜、望远镜和光谱仪。我们对宇宙的大部分了解都来自光带给我们的信息。我们对原子结构的了解主要来自观察它们发出的辐射。因此，光对我们来说非常重要，当光完全没有时，人类什么都看不见。

The history of elucidating the nature of light can be traced back to the beginning of human being. In ancient China, India and Greek, "light" is one of the fundamental "subtle" elements such as Water, Wood, Metal, Fire, and Earth which compose of everything (that should not be confused with the ordinary meaning of these terms). In about , Euclid studied the properties of light. Euclid postulated that light travelled in straight lines and he described the laws of reflection and studied them mathematically. René Descartes (15961650) held that light was a mechanical property of the luminous body and assumed that light behaved like a wave.
阐明光的本质的历史可以追溯到人类的起源。在古代中国、印度和希腊语中，“光”是构成万物的基本“微妙”元素之一，如水、木、金、火和土（不应与这些术语的普通含义混淆）。在大约 ，欧几里得研究了光的性质。欧几里得假设光是直线传播的，他描述了反射定律并用数学方法研究了它们。勒内·笛卡尔（René Descartes，15961650）认为光是发光体的机械特性，并假设光的行为类似于波。

Isaac Newton stated in 1675 that light was composed of corpuscles (particles of matter) which were emitted in all directions from a source. Newton's theory could be used to predict the reflection of light, but could only explain refraction by incorrectly assuming that light accelerated upon entering a denser medium because the gravitational pull was greater. Although there were some outstanding questions, Newton's view was accepted by many scientists and popularized by his followers for over 100 years.
艾萨克·牛顿（Isaac Newton）在1675年指出，光是由微粒（物质粒子）组成的，这些微粒从源头向各个方向发射。牛顿的理论可以用来预测光的反射，但只能通过错误地假设光在进入密度较大的介质时加速来解释折射，因为引力更大。尽管存在一些悬而未决的问题，但牛顿的观点被许多科学家所接受，并被他的追随者推广了 100 多年。

Christiaan Huygens in 1678 proposed that light was emitted in all directions as a series of waves in a medium. As waves are not affected by gravity, it was assumed that they slowed down upon entering a denser medium. The wave theory predicted that light waves could interfere with each other like sound waves, and that light could be polarized, if it were a transverse wave. Early in the 19th century, Young showed that light behaved as waves by means of his famous double-slit diffraction experiment. Young's experiments supported the theory that
克里斯蒂安·惠更斯（Christiaan Huygens）在1678年提出，光在介质中以一系列波的形式向各个方向发射。由于波不受重力的影响，因此假设它们在进入密度较大的介质时会减速。波动理论预言，光波可以像声波一样相互干扰，如果光是横波，光可以偏振。早在19世纪，Young通过他著名的双缝衍射实验证明了光表现为波。Young的实验支持了以下理论：
light consists of waves. A subsequent work by Fresnel soon convinced scientists that light must be a wave, who independently worked out his own wave theory of light. In 1862, James
光由波组成。菲涅耳随后的工作很快使科学家相信光一定是波，他独立制定了自己的光波动理论。1862年，詹姆斯

Clerk Maxwell came up with some brilliant equations, today known as Maxwell's equations and concluded that light was a form of electromagnetic radiation. His theory was later experimentally proven correct by Heinrich Hertz. The weakness of the wave theory was that light waves, like sound waves, would need a medium for transmission. Hence, "ether" (the medium) was invented. However, Michelson-Morley experiments demonstrated that ether does not exist.
书记员麦克斯韦提出了一些绝妙的方程，今天被称为麦克斯韦方程，并得出结论，光是电磁辐射的一种形式。他的理论后来被海因里希·赫兹（Heinrich Hertz）实验证明是正确的。波动理论的弱点是光波和声波一样，需要一种介质来传输。因此，“以太”（媒介）被发明了。然而，迈克尔逊-莫雷实验表明，以太并不存在。

Max Planck in 1900 suggested the idea that black bodies emit light (and other electromagnetic radiation) only as discrete bundles or packets of energy (quanta) and accurately predicted the results of "black-body radiation" observed. Albert Einstein in 1905 developed the particle theory of black-body radiation to explain the photoelectric effect. He suggested that the quantization used by Planck reflects a basic aspect of the reality and light exhibits as a particle. Later, the particle of light was given the name photon with an energy and a momentum P. His ideas were experimentally confirmed by American scientist Robert Millikan a year later. However, his ideas were finally accepted throughout the world only a decade later when the American physicist Compton made theoretical predictions for the scattering of photons from the electron-Compton effect.
马克斯·普朗克（Max Planck）在1900年提出了黑体仅以离散的能量束或能量包（量子）的形式发射光（和其他电磁辐射）的想法，并准确地预测了观察到的“黑体辐射”的结果。阿尔伯特·爱因斯坦（Albert Einstein）在1905年发展了黑体辐射的粒子理论来解释光电效应。他认为，普朗克使用的量子化反映了现实的一个基本方面，而光则以粒子的形式表现出来。后来，光粒子被命名为光子，具有能量 和动量P。一年后，美国科学家罗伯特·米利坎（Robert Millikan）通过实验证实了他的想法。然而，仅仅十年后，当美国物理学家康普顿对电子康普顿效应的光子散射做出理论预测时，他的想法才最终被全世界接受。

Louis de Broglie in 1924 stated that everything has both a particle nature and a wave nature as light. In a word, the modern theory explains the nature of light as wave-particle duality, that is light sometimes exhibits a particle nature and sometimes a wave nature. The quantum theory of light and electromagnetic radiation continued to evolve through the 1920s and 1930s, and culminated with the development during the 1940s of the theory of quantum electrodynamics (QED).
路易斯·德布罗意（Louis de Broglie）在1924年指出，万物都具有粒子性质和光的波动性质。总之，现代理论将光的本质解释为波粒二象性，即光有时表现出粒子性质，有时表现出波动性质。光和电磁辐射的量子理论在 1920 年代和 1930 年代继续发展，并在 1940 年代量子电动力学 （QED） 理论的发展中达到顶峰。

In this chapter, we briefly introduce the geometrical optics and its applications, and use the knowledge of wave motion in chapters 13 and 14 to treat interference, diffraction, and polarization of light.
在本章中，我们简要介绍了几何光学及其应用，并利用第13章和第14章中的波运动知识来处理光的干涉、衍射和偏振。

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Geometrical optics describes light propagation in terms of "rays". A beam of light from its source, on common experience, travels in a straight-line path at a finite speed in a homogeneous medium until it counters a boundary between two different materials. When a beam of light strikes a boundary, it is reflected from that boundary or passes into the material on the other side of the boundary or partially does both. These phenomena are reflection and refraction. The ray approximation is used to represent beams of light. That is a ray of light is an imaginary line drawn along the direction of travel of the light beam. The "ray" is used to approximately model how light will propagate. Light rays are defined to propagate in a rectiline-
几何光学用“射线”来描述光的传播。根据共同的经验，来自其光源的光束在均匀介质中以有限的速度沿直线路径传播，直到它与两种不同材料之间的边界相抵触。当一束光照射到边界时，它会从该边界反射或进入边界另一侧的材料，或者部分地两者兼而有之。这些现象是反射和折射。射线近似用于表示光束。也就是说，一束光是沿着光束的行进方向绘制的一条假想线。“光线”用于近似模拟光的传播方式。光线被定义为在直线中传播
ar path as far as they travel in a homogeneous medium. Rays bend (and may split in ) the interface between two dissimilar media. A light ray is a line or curve that is perpendicular
AR路径，只要它们在均匀介质中传播。光线弯曲（并可能分裂 ）两种不同介质之间的界面。光线是一条垂直的线或曲线
to the light's wavefronts.
到光的波前。

Light is the fastest thing observed in universe. The speed of light in a vacuum is exactly , and approximately , labeled as , a physical constant. When light enters a material, it slows down. For example, light travels about slower in water than it does in a vacuum, while in diamonds, it travels at about half the speed it does in a vacuum. The index of refraction of medium is introduced to measure the speed of light in that medium, defined as
光是宇宙中观测到的最快的东西。真空中的光速正好 是 ，并且近似 于 ，标记为 ，一个物理常数。当光线进入材料时，它会减慢速度。例如，光在水中的传播 速度比在真空中慢，而在钻石中，它的传播速度大约是真空中的一半。引入介质的折射率来测量该介质中的光速，定义为

where is the speed of light in the material. Eq. (15-1) implies that the index of vacuum is taken as unity " 1 ", and the index of refraction of air differs little from unity and in most situations it is considered to be unity without significant difference. As an example, the index of refraction of water is 1.333 , meaning that light travels 1.333 times faster in a vacuum than it does in water. Index of refraction of medium 2 relative to medium 1 is equal to the ratio of the speed of light in the first medium to the speed of light in the second medium
其中 是材料中的光速。式（15-1）意味着真空的折射率被取为单位“1”，空气的折射率与单位差别不大，在大多数情况下被认为是单位，没有显著差异。例如，水的折射率为 1.333 ，这意味着光在真空中的传播速度是水中的 1.333 倍。介质 2 相对于介质 1 的折射率等于第一介质 中的光速与第二介质 中的光速之比

When light goes from one transparent medium such as water or glass to another with different optical properties, usually there is a reflected bean as well as a refracted one as shown in Fig. 15-1, in which the ray 1 represents the incident light, the ray 2 reflected light and the ray 3 refracted light. The incident beam of light in the medium of refractive index encounters the interface (assuming to be a plane) between two media. Part of the light is reflected by the interface to form the ray 2 (reflected light), traveling as if the original beam had bounced from the interface. The rest of the light passes through the interface and into the medium of refractive index , forming the ray 3 (refracted light). The travel of light through an inter face between two media is called refraction. Unless an incident beam of light is perpendicular to the interface, the directions of reflected light and refracted light have some changes with respect to the direction of incident light. For this reason, the beam is said to be bent by refraction.
当光从一种透明介质（如水或玻璃）传播到另一种具有不同光学特性的介质时，通常有反射豆和折射豆，如图15-1所示，其中射线1代表入射光，射线2代表反射光，射线3代表折射光。折射率 介质中的入射光束遇到两个介质之间的界面（假设是一个平面）。部分光被界面反射形成射线 2（反射光），传播时就像原始光束从界面反射一样。其余的光穿过界面进入折射率 介质，形成射线3（折射光）。光通过两种介质之间的界面的传播称为折射。除非入射光束垂直于界面，否则反射光和折射光的方向相对于入射光的方向会有一些变化。出于这个原因，据说光束因折射而弯曲。

The dashed line is the normal, a line perpendicular to the surface at the point where the ray hits. The plane containing the incident ray and the normal is called the plane of incidence, which is in the plane of the page in Fig. 15-1. The angle between the normal and incident is called the angle of incidence, the angle between the normal and reflected ray is the angle of reflection and the angle of between the normal and refracted ray is the angle of refraction. These angles satisfy law of reflection and law of refraction as follow.
虚线是法线，是一条垂直于光线照射点表面的线。包含入射光线和法线的平面称为入射平面，位于图 15-1 页面的平面内。法线和入射 线之间的角度 称为入射角，法线和反射线之间的夹角 是反射角，法线和折射线之间的夹角 是折射角。这些角度满足反射定律和折射定律，如下所示。

Law of reflection: (1) The angle of reflection is equal to the angle of incidence. (2) The in cident ray, the reflected ray, and the normal lie in the plane of incidence.
反射定律：（1）反射角等于入射角。（2）入 射光线、反射光线和法线位于入射平面。

Law of refraction: (1) The incident ray, the refracted ray, and the normal lie in the plane of incidence. (2) The angle of refraction is related to the angle of incidence by
折射定律：（1）入射光线、折射光线和法线位于入射平面。（2）折射角与入射角的关系为：

Law of refraction is called Snell's law. Eq. (15-3) can be rewritten as
折射定律称为斯涅尔定律。式（15-3）可以改写为

Eq. (15-4) implies that for a given angle of incidence, the value of depends on the relative refraction index
方程（15-4）意味着对于给定的入射角，入射角 的 值取决于相对折射率

Fig. 15-1 Schematic diagram of reflection and refraction of light . For example, if (that is ), is less than , and this means that refraction bends the light beam away from the undeflected direction and toward the normal.
图15-1 光 的反射和折射示意图 .例如，如果 （即 ）， 则小于 ，这意味着折射使光束偏离未偏转方向并朝向法线。

If is less than is greater than . In this case, refraction bends the light beam away from the undeflected direction and away from the normal. For the case, if the angle of incidence is greater than the critical angle
如果 小于 大于 。在这种情况下，折射会使光束偏离未偏转的方向并远离法线。对于这种情况，如果入射角大于临界角

for which the angle of refraction is , there is no refracted light and all the light is reflected. This effect is called total reflection. Total reflection has found many applications such as in optical fiber communications and in medical technology.
对于折射角为 ，没有折射光，所有的光都被反射。这种效应称为全反射。全反射在光纤通信和医疗技术等领域得到了许多应用。

There are many application examples of the refraction and the total reflection in everyday life. For example, one can see a fish moving in water near to lakeside and find that the apparent depth of the fish in water is less than the actual depth as shown in Fig. 15-2. Moreover, one cannot find the fish far from lakeside. The law of refraction and total reflection can explain the daily experience.
折射和全反射在日常生活中有许多应用实例。例如，可以看到一条鱼在靠近湖边的水中移动，发现鱼在水中的表观深度小于实际深度，如图 15-2 所示。此外，人们在远离湖边的地方找不到鱼。折射定律和全反射定律可以解释日常经验。

Fig. 15-2 Example of total reflection
图15-2 全反射示例

The laws of reflection and refraction can be used to predict the deflection of light rays and the formations of images for mirrors and lenses.
反射定律和折射定律可用于预测光线的偏转以及镜子和透镜图像的形成。

Example 15-1 Find the critical angle of total reflection for a water-air boundary if the index of reflection of water is 1.33 .
例 15-1 求水-空气边界的全反射临界角，如果水的反射率为 1.33.

Solution From Eq. (15-5), we find the critical angle to be
解 从方程（15-5）中，我们发现临界角为

The careful measurements have shown that the index of refraction in anything but vacuum
仔细的测量表明，除真空外，其他任何东西的折射率
nds on the wavelength of light. The dependence of the index of refraction on wavelughe depends on the wavelength of light. The dependence of the index of refraction on wavelengum
光波长上的NDS。折射率对波面的依赖性取决于光的波长。折射率对波光的依赖性
indicates that when light is refracted by a surface of material, the angle of refraction depends
表示当光被材料表面折射时，折射角取决于
on the wavelength of light. This implies that a very narrow beam of white light will be on the wavelength of light. This implies that a very narrow beam of white light will be spreads out by the refraction such as passing through a prism as shown in Fig. 15-3. This separation of white light into its component colors is called chromatic dispersion. The index of refraction for a material usually decreases with increasing wavelength so that violet light refracts more for
在光的波长上。这意味着一束非常窄的白光将出现在光的波长上。这意味着一束非常窄的白光将通过折射（例如通过棱镜）散开，如图 15-3 所示。这种将白光分离成其成分颜色的现象称为色散。材料的折射率通常随着波长的增加而降低，因此紫光的折射率更高
red light when passing from air into a material.
从空气进入材料时发出红光。

Fig. 15-3 Schematic diagram of chromatic dispersion
图15-3 色散示意图

The dispersion of light is demonstrated most vividly in nature through the formation of a rainbow, often seen by an observer positioned between the Sun and a rain shower.
光的色散在自然界中通过彩虹的形成最生动地表现出来，通常由位于太阳和阵雨之间的观察者看到。

A mirror is a surface that can reflect a beam of light in one direction instead of either scattering it widely in many directions or absorbing it. When light falls on a rough, opaque surface, the incident light is scattered in all directions. If the surface is so smooth that there is little random scattering, the surface is said to be polished and can be used as a mirror. In the following, we use the law of reflection to study the formation of images of an object reflected by the plane mirror, the concave spherical mirror and the convex spherical mirror.
镜子是一种可以在一个方向上反射光束的表面，而不是在多个方向上广泛散射或吸收它。当光线落在粗糙、不透明的表面上时，入射光会向各个方向散射。如果表面非常光滑，几乎没有随机散射，则称该表面经过抛光，可以用作镜子。在下文中，我们利用反射定律来研究平面镜、凹面球面镜和凸面球面镜反射的物体图像的形成。

A point on the surface of an object can be considered as a light source. Generally a source of light can be considered a point source if the resolution of the imaging instrument is too low to resolve its size, or if the object is at a very great distance.
物体表面的点可以被视为光源。通常，如果成像仪器的分辨率太低而无法分辨其尺寸，或者物体距离非常远，则可以将光源视为点光源。

A plane mirror is a mirror with a planar reflective surface. For light rays striking a plane mirror, the angle of reflection equals the angle of incidence. This implies that images of object are upright and the same distance behind the mirror as in front of the mirror. The image size is the same as the object size. That is the magnification of a flat mirror is equal to one. Images are classified as real or virtual. A real image is one in which light actually passes through the
平面镜是具有平面反射面的镜子。对于照射到平面镜的光线，反射角等于入射角。这意味着物体的图像是直立的，镜子后面的距离与镜子前的距离相同。图像大小与对象大小相同。也就是说，平面镜的放大倍率等于一。图像分为真实或虚拟。真实图像是光线实际穿过
image point and a virtual image is one in which the light does not pass through the image point but appears to come (diverge) from that point.
像点和虚拟映像是光线不穿过像点但似乎从该点来（发散）的光。

As shown in Fig. 15-4 (a), when a point object is placed in frond of a plane mirror and sends light in all directions, some of rays are reflected. To an observer in front of the mirror all the reflected rays appear to come from the point I behind the mirror. The observer therefore sees a bright spot which appears to be behind the mirror and which we call the image of the point . The image behind the mirror is a virtual image because the light rays do not actually come from . The distance is the object distance and the distance is the image distance.
如图15-4（a）所示，当将点物体 放置在平面镜的叶子中 并向各个方向发送光时，部分光线被反射。对于镜子前的观察者来说，所有反射的光线似乎都来自镜子后面的点 I。因此，观察者会看到一个亮点，它似乎在镜子后面，我们称之为点 的图像。镜子后面的图像是虚拟图像，因为光线实际上不是来自 。距离 是物体距离，距离 是图像距离。

The image of the object with height of can be found by graphical ray tracing as shown in Fig. 15-4 (b). Two rays start from point and then reflect from mirror. One follows a horizontal path to the mirror, and reflects back on itself. The second ray follows the oblique path and reflects. An observer to the left of the mirror would trace the two reflected rays back to the point from which they appear to have originated, that is the image of point . Because triangles and are identical, the image is far behind the mirror as the object in front, and the object height equals the image height . Lateral magnification is defined as follows
高度为 的 物体 的图像可以通过图形光线追踪找到，如图 15-4 （b） 所示。两条光线从点 开始，然后从镜子反射。一个人沿着一条水平路径 来到镜子前，并反射回自己。第二条光线沿着倾斜路径 反射。镜子左侧的观察者将追踪两条反射光线，追溯到它们似乎起源的点 ，即点 的图像。因为三角形 和 是相同的，所以图像远远落后于镜子，就像前面的物体一样，物体高度 等于图像高度 。横向放大倍率 定义如下

For a plane mirror, because .
对于平面镜， 因为 .

(a)

(b)

Fig. 15-4 Formation of image by reflection from a plane mirror
图15-4 平面镜反射图像的形成

However, the law of reflection also implies that the image is a laterally-inverted "mirror image" of the object. If a person is reflected in a plane mirror, the image of his right hand appears to be the left hand of the image.
然而，反射定律也意味着图像是物体的横向倒置的“镜像”。如果一个人在平面镜中反射，他的右手的图像似乎是图像的左手。

A curved mirror is a mirror with a curved reflective surface, which may be either convex or concave. Most curved mirrors have surfaces that are shaped like part of a sphere, but other shapes are sometimes used in optical devices.
曲面镜是具有曲面反射面的镜子，可以是凸面或凹面。大多数曲面镜的表面形状像球体的一部分，但其他形状有时用于光学设备。

A concave spherical mirror is part of a spherical shell with its inner surface polished as shown in Fig. 15-5 (a). The mirror has radius of curvature , and its center of curvature is at
凹面球面镜是球面壳的一部分，其内表面经过抛光处理，如图15-5（a）所示。镜子有曲率半径 ，它的曲率中心在
point . Point (vertex) is the center of the spherical segment, and a line drawn from is called the principal axis of the mirror. When the rays parallel to the principal axis reach, concave mirror, those near the central axis are reflected through a common point which is called the focal point (or focus) of the mirror (a real focal point) and its distance from the cen. ter of the mirror is the focal length of the mirror. That is
点 。点 （顶点）是球面线段的中心，从 中绘制的线称为镜子的主轴。当平行于主轴的光线到达凹面镜时，靠近中心轴的光线通过一个公共点反射，该公共点 称为镜子的焦点（或焦点）（真正的焦点），它与镜子的厘米的距离是镜子的焦距。那是

If an incident ray of light passes through the focus , it is reflected back in the direction of principal axis.
如果入射光线穿过焦点 ，它会在主轴方向上反射回来。

(a)

(b)

Fig. 15-5 (a) The concave mirror and (b) formation of image
图15-5 （a）凹面镜和（b）图像的形成

As shown in Fig. 15-5(b), an object is placed before a concave mirror, we can locate an image by graphical ray tracing. The ray parallel to the principal axis is reflected back through , and the ray strikes the mirror at point and is reflected back parallel to the principal axis. The ray comes to the spherical surface along the radius and is reflected directly back on itself. The is the image of object and it is a real image since the rays of light come from the . In this case, the image is inverted. Let and denote the distances of object and image respectively. They are related by a simple formula which can be derived easily
如图15-5（b）所示，将物体 放置在凹面镜之前，我们可以通过图形光线追踪来定位图像。平行于主轴的光线 被反射 回来，光线在点 处照射到镜子上，并平行于主轴反射回来。光线 沿半径到达球面，并直接反射回自身。这是 物体 的图像，它是真实的图像，因为光线来自 .在这种情况下，图像是反转的。让 和 分别表示物体和图像的距离。它们通过一个简单的公式相关联，可以很容易地推导出来

This equation applies to any concave, convex, or plane. For a convex or plane mirror, only a virtual image can be formed, regardless of the object's location on the central axis. For a concave mirror, different image types can be formed and depend on the distance between the object and the mirror. Note, according to Eq. (15-8), that an object and it's image are conjugate.
该方程适用于任何凹面、凸面或平面。对于凸面镜或平面镜，无论物体在中心轴上的位置如何，都只能形成虚拟图像。对于凹面镜，可以形成不同的图像类型，并取决于物体与镜子之间的距离。请注意，根据方程（15-8），物体和它的图像是共轭的。

For a convex mirror, we find that the parallel rays are no longer reflected through a common point. Instead, they diverge. As shown in Fig. 15-6, the convex mirror is a diverging mir ror because rays initially parallel diverge after reflection. A ray of light parallel to the principal axis is reflected away from the principal axis, and the reflected ray is extended backward to
对于凸面镜，我们发现平行光线不再通过公共点反射。相反，他们分道扬镳。如图15-6所示，凸面镜是一个发散的mir ror，因为最初平行的光线在反射后会发散。平行于主轴的光线被反射到远离主轴的地方，反射的光线向后延伸至
cross the principal axis as point , the principal focus for this convex mirror. The principal focus is virtual. If we place an object , an erect virtual image is formed and reduce in size. The image on a convex mirror is always virtual, diminished, and upright. Eq. (15-8) can be used for both concave and convex mirrors if we are careful to use the proper signs for various quantities shown in Table 15-1.
以主轴为点 ，是这个凸面镜的主要焦点。主要关注点是虚拟的。如果我们放置一个物体 ，就会形成一个直立的虚拟图像 并缩小尺寸。凸面镜上的图像始终是虚拟的、缩小的和直立的。方程（15-8）既可以用于凹面镜，也可以用于凸面镜，如果我们小心地对表15-1中所示的各种量使用适当的符号。

Fig. 15-6 The convex mirror and formation of image
图15-6 凸面镜与图像的形成

Table 15-1 Sign conventions for mirrors
表 15-1 镜像签名约定

The lateral magnification can be proved to be
可以证明横向放大倍率为

By convention, the lateral magnification includes a plus sign when the image orientation is that of the object and a minus sign when the orientation is opposite that of the object. The imageforming characteristics of curved mirrors obviously determine their uses.
按照惯例，当图像方向为物体的方向时，横向放大倍率包括加号，当方向与物体的方向相反时，横向放大倍率包括减号。曲面镜的成像特性显然决定了它们的用途。

Example 15-2 An object of height is placed from a convex mirror with a focal length of . Find
例 15-2 从焦距为 的 凸面镜放置 高度 物体。找到

(1) the position of the formed image;
（1）形成图像的位置;

(2) the magnification of the mirror.
（2）镜子的放大倍率。

Solution (1) Because the mirror is convex, its focal length is negative. According to Eq. (15-8), we have
解决方案（1）因为镜子是凸面的，所以它的焦距是负的。根据式（15-8），我们有

The negative value of indicates that the image is virtual and behind the mirror.
负值表示 图像是虚拟的，并且在镜子后面。

(2) The magnification of mirror is
（2）镜子的放大倍率

Hence, the image is upright.
因此，图像是直立的。

We now turn from images formed by reflections through spherical surfaces of transparent materials, such as glass. Consider two transparent media with indices of refraction and , where the
现在，我们从反射形成的图像转向透明材料（如玻璃）的球面。考虑两个具有折射率 的透明介质，其中
boundary between the two media is a spherical surface of radius . At the point of refraction of each ray, the normal to the refracting surface is a radial line through the center of curvature. Because of refraction, the ray bends toward the normal if it is entering a medium of greater index of refraction, and away from the normal if it is entering a medium of lesser index of refraction.
两种介质之间的边界是半径为球面 的球面。在每条光线的折射点，折射面的法线是一条穿过曲率中心的径向线。由于折射，如果光线进入折射率较大的介质，则光线会向法线弯曲，如果光线进入折射率较低的介质，则光线会偏离法线。

Fig. 15-7 An Image formed by refraction through a spherical surface
图15-7 通过球面折射形成的图像

Consider a ray from the point source in medium 1 which is incident upon a spherical surface (convex) of medium 2 with center of curvature at as shown in Fig. 15-7. A line drawn from to the center of the segment of spherical surface is called the principal axis of the system. Ray is refracted at surface and an image is formed. According to law of refraction, we have
考虑来自介质 1 中点源 的光线，该光线 入射到介质 2 的球面（凸面），曲率中心如 图 15-7 所示。从 球面段中心 绘制的线称为系统的主轴。光线 在表面折射并形成图像。根据折射定律，我们有

It is easy to prove that
很容易证明这一点

For the paraxial rays, i. e., rays which make small angles with the axis, their sine, their tangent and themselves are essentially equal. Hence we have
对于近轴射线，即与轴成小角度的射线，它们的正弦、切线和它们本身基本上是相等的。因此，我们有

Then, by Eq. (15-10), we have
然后，通过方程（15-10），我们有

Note that real images are formed on the side of surface opposite the side from which the light comes. This is in contrast with mirrors. Eq. (15-12) applies to a variety of circumstances: concave convex and plane surfaces ( approaches infinity). The sign convention for spherical refracting surface is summarized in Table 15-2.
请注意，真实图像是在与光线来自的一侧相对的表面一侧形成的。这与镜子形成鲜明对比。方程（15-12）适用于各种情况：凹面凸面和平面（ 接近无穷大）。球面折射面的符号约定总结于表15-2中。

Table 15-2 Sign conventions for refracting surface
表15-2 折射面的符号约定

Furthermore, the lateral magnification of a refracting surface is
此外，折射表面的横向放大倍率为

Our knowledge of how light rays are bent when they pass from one material to another can now be applied to lenses, which are the basic elements of most optical instruments. A lens is a transparent object with two refracting surfaces whose central axes coincide. The common central axis is the principal axis of the lens. Simple lenses are bounded by faces which are small sections of sphere. When a lens is surrounded by air, light refracts from the air into the lens, crosses through the lens, and then refracts back into the air. Each refraction can change the direction of travel of the light. A lens that causes light rays initially parallel to the central axis to converge is called a converging lens. If, instead, it causes such rays to diverge, the lens is a diverging lens. Fig. 15-8 shows six possible types of spherical lenses with their name: double-convex, plano-convex, plano-concave, concavo-convex, convexo-concave, and doubleconcave.
我们关于光线从一种材料传递到另一种材料时如何弯曲的知识现在可以应用于透镜，这是大多数光学仪器的基本元件。透镜是一个透明物体，具有两个中心轴重合的折射面。公共中心轴是透镜的主轴。简单的透镜由面包围，面是球体的一小部分。当镜头被空气包围时，光线从空气折射到镜头中，穿过镜头，然后折射回空气中。每一次折射都可以改变光的行进方向。使最初平行于中心轴的光线会聚的透镜称为会聚透镜。相反，如果它导致这种光线发散，则透镜就是发散透镜。图15-8显示了六种可能的球面透镜类型，其名称为：双凸透镜、平凸透镜、平凹透镜、凹凸透镜、凸凸透镜和双凸透镜。

Fig. 15-8 Six possible types of spherical lenses
图15-8 球面透镜的六种可能类型

A thin lens is a lens in which the thickest part is thin compared to the object distance , the image distance and the radii of curvature and of the two surfaces of lens. The thin lens approximation ignores optical effects due to the thickness of lenses and simplifies ray tracing calculations. Rays which approach a converging lens parallel to the principal axis are deviated so that they pass through a common point, the principal focus, on the principal axis. A lens has two principal foci, one for light incident from the left, which we denote by , and the second for light incident from the right. For thin lenses the principal foci are equidistant from the optical center of the lens as shown in Fig. 15-9. The focal length of a thin lens with index of refraction surrounded by air is given by the Lensmaker's equation
薄透镜是指与物距 、像距 、曲率 半径和 透镜的两个表面相比，最厚部分较薄的透镜。薄透镜近似忽略了由于透镜厚度引起的光学效应，并简化了光线追踪计算。接近平行于主轴的会聚透镜的光线会偏离，因此它们会穿过主轴上的一个公共点，即主焦点。透镜有两个主焦点，一个用于从左侧入射的光，我们用 表示，第二个 用于从右侧入射的光。对于薄透镜，主焦点与透镜的光学中心等距，如图 15-9 所示。折射率被空气 包围的薄透镜的焦距 由透镜制造商方程给出

where and are the radii of curvature of the two surfaces. Here, is taken to be positive if the first surface is convex, and negative if the surface is concave. The signs are reversed for the back surface of the lens: is positive if the surface is concave, and negative if it is convex. The line is called as principal optical axis, which passes through the center of curvature of each surface. The front and rear (or back) focal planes are defined as the planes which
其中 和 是两个曲面的曲率半径。在这里， 如果第一个表面是凸的，则认为是正的，如果表面是凹的，则认为为负。镜片背面的符号相反： 如果表面是凹的，则为正，如果表面是凸的，则为负。这条线 被称为主光轴，它穿过每个表面的曲率中心。前后（或后）焦平面定义为

(a)

(b)

Fig. 15-9 The foci of thin lenses: (a) double-convex; (b) double-concave
图15-9 薄透镜的焦点：（a）双凸;（b） 双凹

are perpendicular to the principal optical axis and pass through the front and rear focal points, The line through the center of thin lens in Fig. 15-10 is defined as the secondary optical axis.
垂直于主光轴并穿过前后焦点，图15-10中穿过薄透镜中心的线 定义为次光轴。

Fig. 15-10 Formation of a real image by a converging lens
图15-10 聚焦透镜形成真实图像

We shall also consider only light rays that make small angles with the central axis. That is the paraxial approximation. Take a converging lens as an example. As shown in Fig. 15-10, an object outside the focus is a distance from the converging lens and an image is formed at a distance from the lens, which in this case is real and inverted. The image can be located by tracing of three standard rays:
我们还将只考虑与中轴成小角度的光线。这就是近轴近似。以会聚透镜为例。如图15-10所示，焦点 外的物体 与会聚镜头有一段距离 ， 在距离镜头一定距离处形成图像 ，在这种情况下，图像是真实的和倒置的。可以通过追踪三条标准光线来定位图像：

(1) A ray from parallel to the principal axis is deviated so that it passes through the principal focus of a converging lens (or diverge as though it came from the principal focus of a diverging lens).
（1） 平行 于主轴的光线偏离，使其穿过会聚透镜的主焦点（或发散，就好像它来自发散透镜的主焦点一样）。

(2) The ray from which approaches the lens along the line through the principal focus is deviated so that it leaves the lens parallel to the principal axis.
（2）沿线通过主焦点 接近透镜的光线 偏离，使透镜平行于主轴。

(3) A ray through the optical center of lens passes through the lens undeviated.
（3）光线通过透镜的光学中心，不偏斜地穿过透镜。

For a thin lens, in the paraxial approximation, the object and image distances are lated by the equation
对于薄透镜，在近轴近似中，物体 和图像 距离 由方程表示

which is called as the thin-lens equation, and can be used with both converging and diverging lenses. The sign convention of object distance, image distance and focal distance used in this text is listed in Table 15-3.
这被称为薄透镜方程，可用于会聚透镜和发散透镜。本文中使用的物体距离、图像距离和焦距的符号约定如表15-3所示。

Table 15-3 Sign Convention for Thin Lenses
表15-3 薄透镜的标志约定

The lateral magnification is
横向放大倍率为

which is the same as Eq. (15-10) for magnification by a mirror.
这与方程（15-10）相同，用于镜子放大。

Fig. 15-11 Formation of a virtual image by a converging lens
图15-11 通过会聚透镜形成虚拟图像

When an object is inside the principal focus of a converging lens, the image formed is erect, virtual, and enlarged in Fig. 15-11. In this case, the image distance is negative. When an object is placed in front of a system of lenses such as two lenses whose principal axes coincide, the positive of the final image can be calculated by repeated use of the lens equation. The object for the second lens is the image formed by the first. If there is a third lens, the image formed by the second lens acts as its object. The final image formed by a very complicated optical system can be located by successive application of the lens (or mirror) equation. Great care must be taken to assign the proper sign to each distance.
当物体位于会聚透镜的主焦点内时，形成的图像是直立的、虚拟的和放大的，如图 15-11 所示。在这种情况下，图像距离为负。当一个物体被放置在镜头系统（例如主轴重合的两个镜头）的前面时，可以通过重复使用镜头方程来计算最终图像的正值。第二个镜头的对象是第一个镜头形成的图像。如果有第三个镜头，则由第二个镜头形成的图像充当其对象。由非常复杂的光学系统形成的最终图像可以通过连续应用透镜（或反射镜）方程来定位。必须非常小心地为每个距离分配适当的标志。

Example 15-3 An object is placed from a thin diverging lens of focal length . Find the image distance and the lateral magnification.
例 15-3 将物体放置 在焦距 的薄发散透镜上。找到图像距离和横向放大倍率。

Solution From Eq. (15-15), we have
解 从式（15-15）中，我们有

The lateral magnification is
横向放大倍率为

This result implies that the image is virtual, upright and smaller than the object.
这个结果意味着图像是虚拟的、直立的，并且比物体小。

Mirrors and lenses are widely used to form images in many optical instruments. If we had an ideal lens, every ray of light from any given point on the object would cross at exactly the same point on the image. Further, the image would be similar to the object in every respect.
在许多光学仪器中，反射镜和透镜被广泛用于形成图像。如果我们有一个理想的镜头，那么来自物体上任何给定点的每一束光线都会在图像上的完全相同的点上穿过。此外，图像在各个方面都与物体相似。

Any circle on the object would be a perfect circle on the image. If the object were all in
对象上的任何圆圈都是图像上的完美圆圈。如果对象全部在
plane, the image would be all in one plane. Actually no such ideal lens exists. If most of the
平面，图像将全部在一个平面上。实际上，这种理想的镜头并不存在。如果大多数
rays from a point on the object cross at the desired image point but some of them cross off rays from a point on the object cross at the desired image point but some of them cross off of
来自物体上某一点的光线在所需的图像点交叉，但其中一些射线从物体上的某个点交叉到所需的图像点，但其中一些射线交叉
one side, the image becomes blurred. With some lenses, the image of a rectangular obje on distorted so that it is barrel-shaped and in some case it may look like a pincushion. The impect is fect quality of the images is largely the result of defects in shape and form, and is also the re sult of aberrations: spherical aberration, chromatic aberration.
一侧，图像变得模糊。使用某些镜头时，矩形物体的图像会扭曲，因此它是桶形的，在某些情况下它可能看起来像一个枕头。图像质量的不明显性很大程度上是形状和形式缺陷的结果，也是像差的结果：球面像差、色差。

Spherical aberration results from the fact that when rays of light parallel to the principal axis of a spherical lens (or mirror) pass through zones near its edges, they cross the axis nea. rer the lens than those rays which pass through nearer to the center so that the image is blurred as shown in Fig. 15-12(a). Spherical aberration can be reduced by some methods such as using an adjustable aperture and choosing the proper curvature of the lens.
球面像差是由于当平行于球面透镜（或镜子）主轴的光线穿过其边缘附近的区域时，它们会穿过轴 nea。比那些通过更靠近中心的光线更仔细的镜头，使图像模糊，如图 15-12（a） 所示。球面像差可以通过一些方法减少，例如使用可调节光圈和选择合适的镜头曲率。

(a)

(b)

Fig. 15-12 Schematic Diagrams of (a) spherical aberration and (b) chromatic aberration
图15-12 （a）球差和（b）色差示意图

Chromatic aberration is due to the dependence of the index of refraction on wavelength. When rays of white light parallel to the principal axis pass through an ordinary lens, they are refracted in such a way that different colors are brought to focus at different distances from the lens as shown in Fig15-12(b). As result the image is not sharp and is likely to have colored rings or markings. The chromatic aberration can be corrected by combing a converging lens of crown glass with a diverging lens of flint glass since their dispersions are opposite. For better correction of chromatic aberration, more lenses can be used.
色差是由于折射率对波长的依赖性。当平行于主轴的白光穿过普通透镜时，它们会以这样的方式折射，使不同的颜色在距透镜的不同距离处聚焦，如图15-12（b）所示。因此，图像不清晰，并且可能有彩色环或标记。色差可以通过将皇冠玻璃的会聚透镜与燧石玻璃的发散透镜相结合来校正，因为它们的色散是相反的。为了更好地校正色差，可以使用更多的镜头。

We will see that two light waves emitted by two coherent sources can produce interference pattern. The existence of interference phenomena is perhaps the most convincing evidence that light is a wave since interference cannot be explained other than with waves. In or der to understand the interference of light, you must understand firstly the coherence of light.
我们将看到两个相干源发出的两个光波可以产生干涉图案。干涉现象的存在也许是光是波的最有说服力的证据，因为除了波之外，无法解释干涉。要了解光的干涉，首先要了解光的相干性。

When discussing the superposition of mechanical waves in chapter 13 , we saw that two waves emitted by two coherent sources could produce interference pattern in crossing space. However, no interference pattern would be observed when two light waves emitted by two completely independent light sources. It means that two completely independent light sources are not coherent sources.
在第13章讨论机械波的叠加时，我们看到两个相干源发射的两个波可以在交叉空间中产生干涉图案。然而，当两个完全独立的光源发射两个光波时，不会观察到干涉图案。这意味着两个完全独立的光源不是相干光源。

For common sources of visible light, the fundamental light emission processes owe to individual atoms and these atoms do not act together in a coherent way. The action of light emission by a single atom in a typical case takes about and the emitted light is properly described as a wavetrain that length is less than one meter. No constant phase relationship exists between different wavetrains, even if the wavetrains maintain identical frequencies. Lasers are coherent sources of light since its output is a single-frequency radiation of in-phase parallel waves, whereas incandescent light bulbs and fluorescent lamps are incoherent sources.
对于常见的可见光源，基本的光发射过程归功于单个原子，这些原子不会以连贯的方式共同作用。在典型情况下，单个原子的光发射作用大约 需要，发射的光被正确地描述为长度小于一米的波列。不同波列之间不存在恒定的相位关系，即使波列保持相同的频率也是如此。激光是相干光源，因为它的输出是同相平行波的单频辐射，而白炽灯泡和荧光灯是不相干光源。

How can we obtain coherent light from common sources of light? If light waves emitted by an identical point of a monochromatic source can be divided into two beams by using some arrangements, the two beams are coherent light. For example, we can use the arrangements as shown in Fig. 15-13 to obtain coherent light. In Fig. 15-13 (a), and are two wavelet sources on an identical wavefront emitted by a point of a monochromatic source. This ensures that, by diffraction, the same families of wavetrains fall on slits and . When the phase of the light emitted from changes, this change is transmitted simultaneously to and , the diffraction beams emerging from and are thus coherent. This way to obtain coherent light is called the way of division of wavefront. Fig. 15-13(b) shows another easy way to obtain coherent light. Because the beams and are two reflected light waves that come from the same wavetrain , they are coherent light. This way to obtain coherent light is called the way of division of amplitude.
我们如何从常见的光源中获得相干光？如果使用某些排列方式可以将单色光源的相同点发射的光波分成两束光束，则两束光束是相干光。例如，我们可以使用图 15-13 所示的排列来获得相干光。在图15-13（a）中， 和 是由单色源的一个点 发射的相同波前上的两个小波源。这确保了通过衍射，相同的波列系列落在狭缝 上，并且 .当发出的光的相 位发生变化时，这种变化同时传输到 和 ，从 中 产生的衍射光束是相干的。这种获得相干光的方法称为波前分法。图15-13（b）显示了另一种获得相干光的简单方法。因为光束 和 是来自同一波列 的两个反射光波，所以它们是相干光。这种获得相干光的方法称为振幅除法。

(a)

(b)

Fig. 15-13 Easy ways to obtain coherent light: (a) the way of division of wave front and (b) the way of division of amplitude
图15-13 获得相干光的简单方法：（a）波前分法和（b）振幅分法

Interference is not limited to mechanical waves but is a characteristic of all wave phenomena. The existence of interference phenomena of light is perhaps our most convincing evidence that light is a wave.
干涉不仅限于机械波，而是所有波现象的特征。光干涉现象的存在也许是我们最有说服力的证据，证明光是一种波。

The historical experiment that established the wave theory of light is Young's experiment. In 1801, English physicist Thomas Young performed a famous experiment that demon-
建立光的波动理论的历史实验是杨的实验。1801年，英国物理学家托马斯·杨（Thomas Young）进行了一项著名的实验，该实验使——
strated the wave nature of light by showing two overlapping light waves interfered with each
通过显示两个重叠的光波干扰每个光波来划分光的波动性质
other. The experiment also showed how this phenomenon supports the wave theory of other. The experiment also showed how this phenomenon supports the wave theory of Huch he was able to deduce the wavelength of incident light from his measurement, the first deter. mination of this important quantity.
其他。该实验还表明了这种现象如何支持其他的波浪理论。该实验还表明，这种现象如何支持Huch的波动理论，他能够从他的测量中推断出入射光的波长，这是第一个威慑。这个重要数量的减值。

Fig. 15-14 (a) Young's double-slit experiment and (b) the interference pattern
图15-14 （a）杨氏双缝实验和（b）干涉图案

Two narrow slits and are equidistant from the source , as shown in Fig. 15-14 (a), As a beam of monochromatic light from reaches and , each slit serves as a new light source producing new wavefront in phase with each other. The sources are said to be coherent. These waves travel out from and producing bright fringes on the viewing screen where constructive interference occurs and dark fringes where destructive interference occurs. Figure 15-14 (b) shows the interference pattern.
两个窄狭 缝与 光源 等距，如图15-14（a）所示，作为一束来自 到达 和 的单色光束，每个狭缝作为新的光源，产生彼此同相的新波前。据说消息来源是连贯的。这些波从 观察屏幕上 传播出去， 并在发生相长干扰的地方产生明亮的条纹，在发生相消干涉的地方产生暗条纹。图15-14（b）显示了干涉模式。

Let us now consider the theoretical conditions necessary for the production of bright and dark fringes. In Fig. 15-14 (a), the separation of the two slits is represented by , and the screen is located at a distance from the slits. Two beams travel the distances and from slits and to the point on the screen. The point is the midpoint between the two slits and line MP makes an angle with axis . The line is drawn so that the length of line equals to . As the distance is much longer than the slit separation , the path difference coming from and is approximately given by
现在让我们考虑产生明亮和黑暗条纹所需的理论条件。在图15-14（a）中，两个狭缝的分离用 表示，并且屏幕位于距狭缝一定距离 的位置。两束光束 从狭缝 到 屏幕上的点 的距离 。该点 是两个狭缝之间的中点，线 MP 与轴成一个角度 。绘制线 条，使线 的长度等于 。由于距离 比狭缝间隔长得多 ，因此来自 和 的路径差 近似为下式

If angle is small enough, then
如果角度 足够小，那么

Therefore, we have 因此，我们有

Constructive interference will occur at when
在 以下情况下将发生建设性干扰

where is the wavelength of incident light. That is bright fringes will occur at
其中 是入射光的波长。也就是说，明亮的条纹将发生在

Destructive interference will occur at when
破坏性干扰将在 以下情况下发生

That is, dark fringes will occur at
也就是说，暗条纹将出现在

The central bright fringe, , is called the zero-order bright fringe. The bright (or dark) fringes, , are the first-order bright (or dark) fringes which are symmetrically located about and so forth. From Eq. (15-17) and Eq. (15-18), the central distance between two adjacent bright (or dark) fringes is
中央明亮条纹 称为零阶明亮条纹。明亮（或暗）条纹是 一阶亮（或暗）条纹，它们对称地位于 2 等位置。根据式（15-17）和式（15-18），两个相邻的亮（或暗）条纹之间的中心距离为

which is also called the fringe spacing.
这也称为条纹间距。

The result of the interference is to produce a series of alternating bright and dark fringes on the screen, which are equally spaced. We call them as interference fringes. Since the spacing is proportional to the wavelength , the separation for red light is wider than that for violet light. When the incident light contains more than one wavelength, the separate interference patterns with different fringe spacing will be superimposed on the screen. For example, when white light is used in Young's double-slit experiment, except for the central fringe, which is white, the bright fringes are a rainbow of colors.
干涉的结果是在屏幕上产生一系列交替的明暗条纹，这些条纹间隔相等。我们称它们为干涉条纹。由于间距 与波长 成正比，红光的间隔比紫光的间隔更宽。当入射光包含多个波长时，具有不同条纹间距的单独干涉图案将叠加在屏幕上。例如，当在Young的双缝实验中使用白光时，除了中央条纹是白色的外，明亮的条纹是彩虹色。

Example 15-4 In Young's experiment, the slit separation is and the slit-screen separation is . The third-order bright fringe is formed at from the central fringe.
实施例15-4 在Young的实验中，狭缝分离为 ，狭缝屏幕分离为 。从 中央边缘形成三级明亮条纹。

(1) What is the wavelength of light used?
（1）使用的光的波长是多少？

(2) Where will the second-order dark fringes appear?
（2）二阶暗条纹会出现在哪里？

(3) What is the spacing of two adjacent bright fringes?
（3）相邻两条明亮条纹的间距是多少？

Solution (1) For the third-order bright fringe, in Eq. (15-17), thus , so that
解（1）对于三阶亮条纹， 在方程（15-17）中，因此 ，使

(2) The displacement of the second-order dark fringe is found by setting in Eq. (15-18) That is
（2）二阶暗条纹的位移由式（15-18）中设置 求得，即

Where the sign " " shows the second-order dark fringes are symmetrically located about central fringe.
符号“ ”表示二阶暗条纹对称地位于中央条纹附近。

(3) From Eq. (15-19), the spacing of two adjacent bright fringes is
（3）由式（15-19）计算，两条相邻的亮条纹的间距为

Example 15-5 A beam of parallel white light ) is used in Young's experiment with the slits separated by a distance . The screen is located at a distance of from the slits.
例15-5 一束平行白光 ）用于Young的实验中，狭缝相隔一段距离 。屏幕位于距狭缝一定 距离的地方。

(1) Where will the second-order bright fringes appear for red light ? Where will the forth-order bright fringes appear for violet light ?
（1）红光 的二阶亮条纹会出现在哪里？紫光 的四阶明亮条纹会出现在哪里？

(2) What is the spacing of two adjacent dark fringes for red light and for violet light, re. spectively? And give the answers again when is changed into .
（2）红光和紫光的两个相邻暗条纹的间距是多少？当然？ 并在更改为 .

Solution (1) According to Eq. (15-17), for the second-order bright fringe of red light, , we have
解（1）根据式（15-17），对于红光的二阶亮条纹， 我们有

For the forth-order bright fringe of violet light, , we have
对于紫光的四阶明亮条纹， 我们有

(2) From Eq. (15-19), the spacing of two adjacent dark fringes for red light is
（2）由式（15-19）可以看出，红光的两条相邻暗条纹的间距为

and the spacing for violet light is
紫光的间距为

When is changed into , we have
当 改成 时，我们有

We can see that the forth-order bright fringes of violet light will coincide with the secondorder bright fringes of red light. The fringe separation for red light is wider than that of for violet light when keeps a constant and the wavelength of incident light keeps unchanged. The wider the is, the narrower the separation is. When we use white light as incident light in Young's experiment, except for the central bright fringe, all fringes on the screen overlap one another and form rainbow color fringes.
我们可以看到，紫光的四阶亮条纹将与红光的二阶亮条纹重合。当红光保持恒定且入射光的波长保持不变时 ，红光的条纹间隔比紫光的条纹间隔更宽。 越宽，间隔越窄。当我们在Young的实验中使用白光作为入射光时，除了中央的亮条纹外，屏幕上的所有条纹都相互重叠并形成彩虹色条纹。

In chapter 13, we have discussed that if mechanical wave incident from a lower density
在第 13 章中，我们讨论了如果机械波从较低密度入射

Fig. 15-15 Lloyd's mirror medium to a higher density medium, the reflected waves from the interface between them will undergo a phase shift or a wave-path shift . This phenomenon also occurs for light waves' reflection, which can be demonstrated by an ar rangement called Lloyd's mirror, as shown in Fig. 15-15.
图15-15 劳埃德镜面介质到更高密度介质时，反射波从它们之间的界面会经历相移 或波路径偏移 。这种现象也发生在光波的反射中，这可以通过称为劳埃德镜的 ar 排列来证明，如图 15-15 所示。

A narrow slit in an opaque screen is irradiated by one beam monochromatic light of wavelength . A plane mirror is located a distance below . The actual source and its vir tual image formed by the reflected light waves from the surface of plane mirror are a pair of herent sources. The fringes formed by interference between the coherent light waves from and can be viewed in the interference zone on screen . When we move to which is in contact
不透明屏幕 中的窄缝被波长 的一束单色光照射。平面镜 位于下方 的距离 。平面镜表面反射光波形成的实际光源 及其视觉图像 是一对 遗传光源。由来自 和 的相干光波之间的干涉形成的条纹可以在屏幕上 的干涉区中看到。当我们移动 到 哪个是联系的
with the edge of the mirror (Fig. 15-16), the fringe nearest the edge is dark. This is, however, difference. We have to recognize that the zeroth fringe is dark indicates that the waves reflected from the surface of mirror have undergone a phase shift of . The experiment of Lloyd's mirror demonstrates that a one-half a wavelength (or a .phase s lifift occurs when light reflects from a medium having an index of refraction higher than that of the medium in which it is originally traveling.
与镜子的边缘（图 15-16）相比，最靠近边缘的边缘是深色的。然而，这是区别。我们必须认识到，第零条纹是暗的，表明从镜子表面反射的波已经发生了相 移。劳埃德镜子的实验表明，当光从折射率高于其最初传播的介质反射时，就会发生半个波长（或相 位 s lifift）。