SPH4U UNIVERSITY PHYSICS

Transcription

1 SPH4U UNIVERSITY PHYSICS THE WAVE NATURE OF LIGHT L Polarization of Light (P ) What is Polarized Light? Polarized sunglasses eliminate the glare of reflected light off the surface of a stream while other sunglasses do not. What is unique about polarized lenses? The answer is based on a specific property of electromagnetic radiation including light. December 12, U4 - Polarization of Light 1 What is Polarized Light? Evidence for this property was first reported in 1669 when the Danish scientist Erasmus Bartholinus observed that a single ray of light separated into two distinct rays while passing through a piece of naturally occurring calcite crystal. December 12, U4 - Polarization of Light 2 1

2 What is Polarized Light? However, to understand the principles behind polarized lenses and the splitting of a ray of light by calcite crystals, you first need to grasp the concept of polarization. December 12, U4 - Polarization of Light 3 Polarization of Light As you know, electromagnetic waves have electric and magnetic fields that are perpendicular to each other and to the direction of propagation. These fields can take many different directions and still be perpendicular to the direction of propagation. It is these directions that determine the polarization of the light. December 12, U4 - Polarization of Light 4 Polarization of Light Thus, a mixture of light with different waves and different directions of propagation is said to be unpolarized light while light waves which vibrate in a single plane are said to be polarized light. And polarization occurs when you block one of the axes of light oscillation. December 12, U4 - Polarization of Light 5 2

3 Polarization of Light UNPOLARIZED LIGHT mixture of light with different waves and different directions of propagation POLARIZED LIGHT light waves which vibrate in a single plane December 12, U4 - Polarization of Light 6 As shown earlier, polarization occurs when you block one of the axes of light oscillation. Polarizing filters, developed in the 1920s, have the ability to selectively absorb all but one orientation of the electric fields in electromagnetic waves, as shown. After light or any electromagnetic wave has passed through such a filter, all of the electric fields lie in one plane and the wave is said to be plane polarized. December 12, U4 - Polarization of Light 7 PLANE POLARIZED LIGHT light that is polarized in one direction that is perpendicular to the direction of propagation December 12, U4 - Polarization of Light 8 3

4 This is similar to generating randomly polarized pulses in a length of rope. Only the pulses that are vertically oriented will make it through a slit in a board that is oriented vertically. All the other waves will be blocked. December 12, U4 - Polarization of Light 9 1. What will happen if a horizontally oriented slit is placed immediately after the vertically oriented slit? December 12, U4 - Polarization of Light What property of waves does this demonstrate? that the waves are transverse and not longitudinal (otherwise, polarization by selective absorption would not work) December 12, U4 - Polarization of Light 11 4

5 3. In each of the following, describe the process involved as light travels from the source to the analyzing filter. bottom none of the light is making it through top component of the light oriented in the slanted plane is making it through (notice the smaller amplitude) December 12, U4 - Polarization of Light 12 Polarization by Reflection In addition, when light or any other electromagnetic wave strikes a surface, the electric fields that are perpendicular to that surface are absorbed and the parallel or horizontal electric fields are reflected. Therefore, the reflected light is polarized parallel to the reflective surface. December 12, U4 - Polarization of Light 13 Polarization of Light POLARIZATION occurs when one of the axes of light oscillation is blocked can occur by: selective absorption (i.e. a filter) reflection (i.e. the light is polarized parallel to the surface) demonstrates that light is a transverse wave (and not longitudinal) December 12, U4 - Polarization of Light 14 5

6 4. How do polarized lenses exclusively absorb glare from the light that is reflected from the surface of a steam or the hood of a car? December 12, U4 - Polarization of Light 15 Polarized lenses in sunglasses are oriented so that they allow only vertically polarized light to pass through and thus absorb most of the horizontally polarized reflected light. So, the fish beneath the water s surface is clearly visible when the bright glare, consisting mainly of horizontally polarized light, is removed. December 12, U4 - Polarization of Light Just before the Sun sets, a driver encounters sunlight reflecting off the side of a building. Will Polaroid sunglasses stop this glare? Explain. no because the reflected light is vertically oriented and so too are the sunglasses December 12, U4 - Polarization of Light 17 6

7 Besides sunglasses, many applications make use of the polarization of electromagnetic waves. For example, materials such as glass and plastic have different indices of refraction for different polarization of light and produce a colourful display when viewed through polarized sunglasses. Amazingly, this effect has practical uses in designing large structures. December 12, U4 - Polarization of Light 18 For example, engineers will often analyze the design of a large structure by constructing a Lucite model and then subjecting the model to various forces. By analyzing the mechanical stresses (visible because of differing indices of refraction), the engineers are able to identify where structural failure might occur and design additional support. December 12, U4 - Polarization of Light 19 APPLICATIONS OF POLARIZERS sunglasses (only allow vertically polarized light through) Lucite models of large structures (to analyze mechanical stresses) the design and production of LCD displays December 12, U4 - Polarization of Light 20 7

8 Optical Activity & LCDs DYK? When linearly polarized light passes through certain materials, the materials rotate the direction of the light's polarization in an effect called optical activity. Materials that are optically active usually contain molecules with a screw-like or helical structure. Liquid crystal displays (LCDs) make use of polarizers to create optical activity. December 12, U4 - Polarization of Light 21 Optical Activity & LCDs DYK? For example, when no voltage is applied, the long, screw-shaped molecules in the liquid crystal rotate the plane of polarization by 90E so that the outgoing light can pass through an output polarizer. December 12, U4 - Polarization of Light 22 Optical Activity & LCDs DYK? However, an applied voltage causes the liquid crystal to stop rotating its plane of polarization. As a result, the light that comes out of the liquid crystal is still polarized at 90E relative to the output polarizer, so no light is transmitted and the display appears dark. December 12, U4 - Polarization of Light 23 8

9 U Check Your Learning TEXTBOOK P.537 Q.2,4,6 WIKI (WAVE NATURE OF LIGHT) O... 4U4 - QUIZ#2 (EM Radiation & Polarization) December 12, U4 - Polarization of Light 24 9