Polarization

Polarization Describe what is meant by polarized light. Describe polarization by reflection. State and apply Brewster’s law. Explain the terms polarizer and analyser. Calculate the intensity of a transmitted beam of polarized light using Malus’ law. Describe what is meant by an optically active substance. Describe the use of polarization in the determination of the concentration of certain solutions. Outline qualitatively how polarization may be used in stress analysis. Outline qualitatively the action of liquid-crystal displays (LCDs).

Polarized waves (transverse only) Vibrations lie in the same plane

Polarized light

Often a plastic called “Polaroid” discovered by a 19 year-old Harvard undergradutae called Edwin Land in 1928 Polarized light

Polarized light

Polarization by reflection

Brewster angle In 1812, Sir David Brewster found experimentally that the reflected ray is 100% polarized when the angle between the reflected ray and the refracted ray is 90°

Brewster’s angle tanθB = n2/n1 Completely polarized reflected ray normal θB η = n1 η = n2

Brewster’s angle tanθB = n2/n1 If ray is incident from air, n1 = 1, so tanθB = n2 Completely polarized reflected ray normal θB η = n1 η = n2

Calculate Brewster’s angle for light incident on water (η = 1.33)

Calculate Brewster’s angle for light incident on water (η = 1.33) tanθB = n2/n1 = 1.33/1 = 1.33 θB = 53.1º

Polarizers and analysers A polarizer (like polaroid) can be used to polarize light

Polarizers and analysers A polarizer can also be used to determine if light is polarized. It is then called an analyser.

Malus’ Law The intensity of polarised light that passes through a polarizer is proportional to the square of the cosine of the angle between the electric field of the polarized light and the angle of the polarizer!

Malus’ law I = Iocos2θ Io Iocos2θ

Optical activity Some substances can change the plane of polarized light. We say they are optically active

Optical activity Sugar solution is optically active. The amount of rotation of the plane of polarization depends on the concentration of the solution.

Stress analysis

Stress analysis Some substances, not normally optically active, become optically active if subject to stresses.

Stress analysis Analysis of the patterns reveals how the stress varies in the material.

Liquid crystal displays

Liquid crystal display

Questions! PHYSICS Pages 278-279 Qs 4, 8, 11, 13, 22.