专业英语第14讲.ppt

1、English in Optoelectronic Information Technology,Chapter 2 Wave Optics,Electromagnetic waves are usually generated by oscillating electric charges, such as the oscillating current in a radio or TV antenna or an electron in an atom. 电磁波通常是由振荡电荷产生，例如收音机、电视天线产生的振荡电流或原子中的电子。 The direction of the oscilla

2、tion determines the orientation of the waves electricfield. In most common light sources, such as a candle flame, the sun, or an incandescent lamp, the many oscillating atoms are randomly oriented. 振动方向决定了波动的电场方向。在大多数普通光源，例如一个烛焰、太阳、或者一个白炽灯，许许多多的振荡原子的方向都是随意地。,2.10 Dispersion,2,Although the electric f

3、ield of each wave produced by a particular atom lies in a single plane, the overall beam of light contains electric fields oscillating in all planesFigure 2.37(a). 尽管每一个特定原子产生的波的电场分布在一个单独的平面，光的整体光束包含了在所有平面内振动的电场。 However, if the oscillating charges are confined to move in only one plane, as is the o

4、scillating current in an antenna, then the entire electric field of the resulting beam oscillates in only that direction Fig 2.37(b). 然而，如果振荡电荷被限定在一个平面内运动，并且天线的振荡电流也被限定在一个平面内，则最终光束的整个电场仅在那个方向振动图2.37（b）。,2.10 Dispersion,3,Such a wave is said to be polarized, and the orientation of the electric field

5、vector is taken as the direction of polarization. 这样一个波被称为是偏振的，并且电场矢量的方向被取为偏振的方向。 The complete wave has a sinusoidally oscillating magnetic field coupled to the electric field, but traditionally we refer to only the electric field to indicate the direction of polarization. 整个光波具有正弦分布的磁场和电场，但是通常我们尽把电

6、场的方向标明为偏振方向。,2.10 Dispersion,4,A useful mechanical analogy for polarized light is the motion of transverse waves along a rope. If you shake a rope up and down, the wave is vertically plane-polarized. 偏振光的一个有用的机械的类比是沿绳子的横波的运动。如果你上下摇动一个绳子，（绳）波将是垂直平面偏振的。 Such a wave can travel through a vertical slit F

7、ig 2.38(a), but not through a horizontal slit. Similarly, if you shake a rope side to side, the wave is confined to a horizontal plane; it can pass through a horizontal slit, but not a vertical one Fig 2.37(b). 这样一个波可以穿过一个垂直的狭缝图2.38（a），但是不能穿过一个水平的狭缝。同样地，如果你水平摇动绳子，波会被限定在一个水平面内；它可以穿过一个水平的狭缝，但是不能穿过一个垂直

8、狭缝图2.37（b）。,2.10 Dispersion,5,It is possible to produce polarized light by filtering ordinary light through a polarizer, which is a material that transmits only those waves that oscillate in a single plane. 采用起振器可以把自然光过滤成为偏振光，起偏器是一种仅传播在一个平面内振动的波的材料。 Several materials have this property, including th

9、e naturally occuring mineral tourmaline. When an ordinary beam of light strikes a tourmaline crystal, part of the light is absorbed and part is transmitted Fig 2.39(a). 几种材料具有这样的特性，包括天然生成的矿物质电气石。当一个自然光照射到一个电气石晶体，部分光被吸收，部分光透射。,2.10 Dispersion,6,If a second tourmaline crystal is placed behind the firs

10、t and aligned in the same orientation, light is transmitted. If the second crystal is rotated by 90 about the axis of the light beam, no light passes through Fig. 2.39(b). 如果再放置一块电气石晶体于第一块的后面并且沿着相同的方向，光线透过。如果第二个晶体关于光轴旋转90，则没有光透过图2.39. We can explain this observation in terms of polarization. The ini

11、tial light beam is unpolarized. We think of it as containing a mixture of random polarizations. 我们可以就偏振来解释这一观察（到的现象）。最初的光是非偏振的。我们认为它包含任意方向的偏振。,2.10 Dispersion,7,When this beam strikes the first crystal, the tourmaline passes only light polarized along the polarization axis of the crystal. The light

12、emerging from the first crystal is completely polarized along the direction shown. 当光线照射到第一片晶体，电气石只透过偏振方向沿着晶体的偏振轴的光。从第一块晶体出射的光完全沿着图示的方向偏振。 When this light falls on the second tourmaline crystal, all the light is absorbed, because the second crystal has been turned so that its polarization axis is 90

13、 away from that of the first crystal. 当光线照射到第二个电气石晶体，所有的光线被吸收，因为第二个晶体的偏振轴与第一个晶体的成90 。,2.10 Dispersion,8,Crystal polarizers like tourmaline are not used much now because of the development of inexpensive plastic polarizers called Polaroids. 像电气石这样的晶体偏振片用的不是很多，因为更便宜的被称为人造偏振片的塑料偏振片的发展。 Synthetic sheet

14、polarizers were invented in 1928 by Edwin H. Land and improved by him ten years later. When viewed individually, these transparent sheets have a neutral grey appearance. 合成片状偏振器是由爱德温.H.兰德在1928年发明的，十年后他又进行了改进。逐个来观察，这些透明的偏振片具有非彩色的灰色外观。,2.10 Dispersion,9,However, when two polarizers overlap with their

15、polarization directions at right angles, the region of overlap looks black Fig.2.40. 然而，当这两个偏振片的偏振方向沿直角重叠，重叠区域看起来是黑色的图2.40。 If the angle between the two Polaroids is made less than 90, the area of overlap is no longer opaque. It gradually gets more and more transparent as the angle decreases to 0. 如

16、果两个偏振片之间的角度小于90，则重叠区域不再是不透明的。随着角度的减小到0，将会越来越透明。,2.10 Dispersion,The intensity of light that passes through two polarizers that are aligned with an angle between their polarization directions is I=Imcos2 (2.11) where Im is the maximum amout of light transmitted when the two polarizers are aligned alo

17、ng the same direction of polarization. 穿过两个偏振片的光的强度，和它们偏振方向的夹角有如下关系： I=Imcos2 (2.11) ，其中Im是当两个偏振片沿着相同的偏振方向排列时透射光的最大值。 Equation (2.11) is known as Maluss Law after its discoverer, who first observed this effect experimentally in 1809. 方程(2.11)被称为马吕斯定律，在1809年马吕斯第一次通过实验发现了这种现象。,2.10 Dispersion,An ideal

18、 polarizer passes 100% of the incident light that is plane-polarized along its polarization direction and 0% of the light that is polarized at 90 to its polarization direction. 一个理想的偏振片沿它们的偏振方向上，可以透过100%的线偏振光，垂直于它们的偏振方向上，完全不透过线偏振光。 Since unpolarized light is a mix of all possible polarizations, an i

19、deal polarizer transmits only 50% of the incident unpolarized light intensity. 由于非偏振光是所有可能的偏振方向的混合，一个理想的偏振片只能透过50%的入射非偏振光。,2.10 Dispersion,Malus also discovered that light becomes polarized upon reflection from glass windows and surfaces of water. 马吕斯同样发现了当光从玻璃窗或者水面反射时变成偏振的。 In 1814, some six years

20、after this discovery, David Brewster (1781-1868) found that at a particular angle of incidence, now called the Brewster angle, polarization of the reflected light is complete, with polarization perpendicular to the plane of incidence (Fig.2.40). 在1814年，当这现象发现后约6年后，大卫.布儒斯特（1781-1868）发现了一个特殊的入射角，现在称为布

21、儒斯特角。反射光的偏振垂直于入射平面的光完全偏振。,2.10 Dispersion,At other angles, the reflected light is only partially polarized, with more light polarized in this direction than in any other. 在其它的角度，反射光只是部分偏振，在这个方向的偏振比其它方向的多。 Brewster found that maximum polarization occurs when the reflected ray and the refracted ray ar

22、e at right angles to each other. By combining Brewsters observation with Snells law, we can find a relation between a materials index of refraction and its Brewster angle. 布儒斯特发现最大偏振发生在当反射光线和折射光线恰好在直角角度。利用斯涅尔定律并结合布儒斯特的观察，我们发现材料的折射率和布儒斯特角之间存在着联系。,2.10 Dispersion,Maximum polarization occurs for r+t=90

23、, where r is the angle of reflection and is the angle of transmission (refraction). For Snells law we can show that nisini= ntsint = ntsin(90-r) = ntcosr 最大的偏振发生在r+t=90，这里r是反射角，而t是透射（折射）角。由Snell定律，我们可以得到nisini= ntsint = ntsin(90-r) = ntcosr 。 This equation reduces to nisini= ntcost When we use the f

24、act that i=r. At this condition of maximum polarization, we can denote the angle of incidence as the Brewster angle B and rewrite this equation as tan B =nt/ni (2.12) 这个公式可以简化成：nisini= ntcost 。这里我们假定了r=t。在这个最大偏振的条件下，我们可以指出这个入射角是布儒斯特角B，并且把这个方程写为tanB=nt/ni.,2.10 Dispersion,This last equation is known

25、as Brewsters law. Note that the Brewster angle depends on the index of refraction of the materials on both sides of the reflecting dielectric surface. 最后一个方程被称为布儒斯特定律。指出了布儒斯特角取决于反射表面的两种电介质材料的折射率。 Sunglasses made from Polaroid sheet are widely used because of their ability to block the glare of specularly reflected sunlight from