9 4 Polarization of light waves Polarization by

9. 4 Polarization of light waves • Polarization by selective absorption • Polarization by reflection • Polarization by scattering • Uses of polarization of light © Manhattan Press (H. K. ) Ltd. 1

9. 4 Polarization of light waves (SB p. 39) Polarization is the restriction of the vibrations in a wave so that the vibrations occur in a single plane. It only occurs in transverse waves, such as electromagnetic waves. © Manhattan Press (H. K. ) Ltd. 2

9. 4 Polarization of light waves (SB p. 39) 1. Polarized light & non-polarized light - A light wave is emitted when an electron orbiting round a nucleus drops from a higher energy level to a lower energy level. - The transition of the electron produces a light wave. © Manhattan Press (H. K. ) Ltd. 3

9. 4 Polarization of light waves (SB p. 39) Polarized light Vertical polarized light wave from an electron orbiting in the vertical plane. © Manhattan Press (H. K. ) Ltd. Horizontal polarized light wave from an electron orbiting in the horizontal plane. 4

9. 4 Polarization of light waves (SB p. 39) Non-polarized light - The light from a source such as candle flame, a filament lamp or the sun is non-polarized. - This is because the light waves are from different atoms whose electrons experiencing the transition are orbiting in different planes in all directions. © Manhattan Press (H. K. ) Ltd. 5

9. 4 Polarization of light waves (SB p. 40) Polarizer A polarizer allows electric field of the light to vibrate in only one direction and simultaneously absorbs all other electric fields vibrating in different directions, resulting in the polarized light. The direction of vibration of the polarized light is known as the polarization axis. © Manhattan Press (H. K. ) Ltd. 6

9. 4 Polarization of light waves (SB p. 40) Polarizer If Io is the intensity of light incident on a polarizer, the intensity of light transmitted by the polarizer is © Manhattan Press (H. K. ) Ltd. 7

9. 4 Polarization of light waves (SB p. 40) Viewing with a polarizer © Manhattan Press (H. K. ) Ltd. 8

9. 4 Polarization of light waves (SB p. 40) Analyser Since the intensity of light (I) is directly proportional to the square of the amplitude of vibration, I (A cosθ)2, the intensity of light emerging from analyser Q is: I = Io cos 2θ © Manhattan Press (H. K. ) Ltd. [Io A 2 ] 9

9. 4 Polarization of light waves (SB p. 41) Variation of intensity If analyser Q is now rotated slowly about the direction of propagation, the intensity of the transmitted light decreases as the angle θ increases from 0°to 90°. When θ = 90°, no light passes through it. The variation of intensity with the angle θis shown below: © Manhattan Press (H. K. ) Ltd. 10

9. 4 Polarization of light waves (SB p. 41) Variation of intensity - If θ is increased further, the intensity increases until a maximum is reached when θ = 180°. - For θ from 180° to 270°, the intensity again decreases and reaches zero when θ = 270°. - For θ from 270° to 360°, the intensity of the transmitted light increases to a maximum until θ = 360°. © Manhattan Press (H. K. ) Ltd. 11

9. 4 Polarization of light waves (SB p. 41) Testing of polarized light A piece of polarizer held between the eyes and a pair of glasses. It can be used to determine whether a beam of light is polarized. © Manhattan Press (H. K. ) Ltd. 12

9. 4 Polarization of light waves (SB p. 41) Testing of polarized light The polarizer is then slowly rotated about the axis of vision. If the intensity of light remains unchanged, then the incident beam is not polarized. If the intensity of the transmitted light varies as the polarizer is rotated and at certain angles of rotation, no light is transmitted, then the light is plane polarized. Go to Example 3 Example 4 © Manhattan Press (H. K. ) Ltd. 13

9. 4 Polarization of light waves (SB p. 43) Polarization by selective absorption If a polarizer is used to select one specific direction of vibration of the wave, then the process is called polarization by selective absorption. © Manhattan Press (H. K. ) Ltd. 14

9. 4 Polarization of light waves (SB p. 43) Polaroid - The modern form of the polaroid is made from a plastic sheet which is highly strained to align the molecules in one direction. - The molecules have long chains which allow the electric field to set the electrons to move freely along the chain. - Therefore, the energy of electromagnetic waves is transformed into internal energy. - The electric field in this direction is thus absorbed. © Manhattan Press (H. K. ) Ltd. 15

9. 4 Polarization of light waves (SB p. 43) Polarization by reflection Brewster’s Law Rotate the polaroid until reaching the minimum intensity and then keep its orientation fixed. Change the viewing angle until the intensity drops to almost zero. This angle of incidence (i) at the reflecting surface satisfies the following equation: tan i = n where n is the refractive index of the material. This is known as Brewster’s Law and i is Brewster’s angle (57º) © Manhattan Press (H. K. ) Ltd. 16

9. 4 Polarization of light waves (SB p. 43) Brewster’s Law From Snell’s Law, refractive index, where r is the angle of refraction. From Brewster’s Law, tani = n when polarization is complete. © Manhattan Press (H. K. ) Ltd. 17

9. 4 Polarization of light waves (SB p. 44) Brewster’s Law Hence, sinr = cosi = sin(90 i ) r = 90 i i + r = 90 That is, The reflected light is completely polarized when the reflected ray and the refracted ray are at right angles. © Manhattan Press (H. K. ) Ltd. 18

9. 4 Polarization of light waves (SB p. 44) Explanation of polarization by reflection © Manhattan Press (H. K. ) Ltd. 19

9. 4 Polarization of light waves (SB p. 44) Polarization of scattering - If light propagates through a gas (or liquid), the electrons in the gas can absorb and re-radiate part of the light. - The absorption and re-radiation of light by the gas is called scattering. - This explains how the sunlight reaching above an observer on the earth is partially polarized as shown. © Manhattan Press (H. K. ) Ltd. 20

9. 4 Polarization of light waves (SB p. 45) Study polarization of scattering in lab. 1. Add a few drops of milk to make a transparent tank of water cloudy. 2. Emit a laser beam (such as He-Ne laser, red light) into the tank. 3. View the scattered light at right angle and other different positions by using a polaroid. 4. Rotate the polaroid and observe the light carefully. © Manhattan Press (H. K. ) Ltd. 21

9. 4 Polarization of light waves (SB p. 45) The results show that the scattered light is plane polarized at positions A and B, i. e. at right angles to the incident ray. Obviously, the light passing straight through the tank at position C remains unpolarized. The light scattered at other positions such as position D are partially polarized. © Manhattan Press (H. K. ) Ltd. 22

9. 4 Polarization of light waves (SB p. 45) Uses of polarization of light Sunglasses use polaroid to reduce the glare from the reflected sunlight. This is because reflected light from a shiny horizontal surface (e. g. the sea) is partially horizontally polarized. The polaroid used has its polarization axis in vertical. Then most of the reflected light will be absorbed by the sunglasses and the intensity of the light entering the eyes is very much reduced. © Manhattan Press (H. K. ) Ltd. 23

9. 4 Polarization of light waves (SB p. 46) Receiving radio wave signals Radio signals used for transmitting information are usually polarized. Three common radio waves in HK broadcasting as follows: AM radio © Manhattan Press (H. K. ) Ltd. FM radio TV 24

9. 4 Polarization of light waves (SB p. 46) AM radio AM bands radio waves are usually vertically polarized. The radio waves set the electrons to vibrate up and down along in the vertical antenna. This creates a high frequency alternating current into the circuit inside the radio. © Manhattan Press (H. K. ) Ltd. 25

9. 4 Polarization of light waves (SB p. 46) FM radio broadcasts in the VHF band (around 100 MHz) and is horizontally polarized. Thus, FM antennas should be horizontal. © Manhattan Press (H. K. ) Ltd. 26

9. 4 Polarization of light waves (SB p. 46) TV aerial TV broadcasts in the UHF band (around 400 MHz). In Hong Kong, the TV signals make use of both vertical and horizontal polarization in order to save bandwidth. Therefore, the direction of the TV antennas is different in different districts. Although there is a set of receivers on an antenna, only the last one is connected to the television cable electrically. Others are actually reflectors that focus the signals on the last receiver. © Manhattan Press (H. K. ) Ltd. 27

9. 4 Polarization of light waves (SB p. 46) Note The receiver and reflectors are parallel to the direction of the electric field of the TV signals while the supporter of the antenna is along the direction of propagation of the waves. © Manhattan Press (H. K. ) Ltd. 28

9. 4 Polarization of light waves (SB p. 46) Determining the concentration of a solution When polarized light passes through a certain solution, the plane of polarization is rotated. The angle of rotation increases as the concentration of the solution increases. This property of rotating the plane of polarization is known as optical activity. © Manhattan Press (H. K. ) Ltd. 29

9. 4 Polarization of light waves (SB p. 46) Determining concentration of a sugar solution – by polarimeter - By viewing the transmitted light through a tube containing sugar solution using a second polaroid which is rotated, the angle through which the plane of polarization has turned can be measured. - Hence, the concentration of the sugar solution can be determined. This device is called a polarimeter. © Manhattan Press (H. K. ) Ltd. 30

9. 4 Polarization of light waves (SB p. 47) Analysing stress If a piece of glass which is strained is placed between two pieces of polaroid with their polarization axes crossed at right angles, a stress pattern which consists of lines of various colours is observed. This is because light of different wavelengths are rotated by different amounts. This effect can be used to analyse some plastic structures like perspex and polythene. Go to © Manhattan Press (H. K. ) Ltd. More to Know 2 31

9. 4 Polarization of light waves (SB p. 47) 3 -D movie In three-dimensional movies, a pair of pictures are taken a short distance apart. The pictures are then projected onto a screen through a pair of projectors fitted with polarized filters. The polarization axes of the filters are at right angles to each other – one horizontal and the other vertical. To the naked eye, the pictures on the screen look blurred. © Manhattan Press (H. K. ) Ltd. 32

9. 4 Polarization of light waves (SB p. 47) 3 -D movie - To see the pictures in 3 -D, the viewer needs to wear glasses made of polaroid as shown. Each eye sees a separate picture, just as in real life. - The brain then interprets the two pictures as a single picture with depth. © Manhattan Press (H. K. ) Ltd. 33

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9. 4 Polarization of light waves (SB p. 42) Q: A beam of unpolarized light passes through polarizers P 1, P 2 and P 3 as shown in the figure. The polarization axis of polarizers P 1 and P 3 are vertical but that of polarizer P 2 is inclined at 30° to the vertical. To obtain the minimum intensity for the emergent beam, through what angle must polarizer P 3 be rotated in the clockwise direction? Solution © Manhattan Press (H. K. ) Ltd. 35

9. 4 Polarization of light waves (SB p. 42) Solution: The plane of polarization of the beam that passes through P 2 is inclined at an angle of 30° to the vertical. For the intensity of the emergent beam from P 3 to be a minimum, the polarization axis of P 3 must be perpendicular to the plane of polarization of the light incident on it. Hence, P 3 must be rotated through (90° + 30°) = 120°. Return to Text © Manhattan Press (H. K. ) Ltd. 36

9. 4 Polarization of light waves (SB p. 42) Q: Two pieces of polarizers are arranged such that their polarization axes are parallel and vertical, as shown in the figure. The intensity of the emergent light is I 0. Through what angle must polarizer Q be rotated so that the intensity of the emergent light decreases to ? Solution © Manhattan Press (H. K. ) Ltd. 37

9. 4 Polarization of light waves (SB p. 42) Solution: When the polarization axis of polarizer Q is at an angle θ to the vertical, the intensity of the emergent light is I = I 0 cos 2θ When cos 2θ= cosθ= θ= 45 © Manhattan Press (H. K. ) Ltd. Return to Text 38

9. 4 Polarization of light waves (SB p. 47) Stress pattern on windscreen of cars Return to Text The curved windscreen of a car is always under strain. Reflected light which is partially horizontally polarized after falling on the windscreen has its plane of polarization rotated. A driver wearing polaroid sunglasses with a vertical polarization axis will be able to observe the stress pattern which is in the form of parallel bands identical to interference fringes. © Manhattan Press (H. K. ) Ltd. 39