This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
- 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.
A polarizer allows electric field of the light to vibrate in only one direction and simultaneously absorbs all other electric fieldsvibrating in different directions, resulting in the polarized light.
The direction of vibration of the polarized light is known as the polarization axis.
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:
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:
The polarizer is then slowly rotated about the axis of vision. If the intensity of lightremains 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.
Testing of polarized lightTesting of polarized light
Polarization by reflectionPolarization by reflection
9.4 Polarization of light waves (SB p. 43)
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)
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.
Determining concentration of a sugar Determining concentration of a sugar solution – by polarimetersolution – 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.
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.
Go to
More to Know 2More to Know 2
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.
The plane of polarization of the beam that passes through P2 is inclined at an angle of 30° to the vertical.
For the intensity of the emergent beam from P3 to be a minimum, the polarization axis of P3 must be perpendicular to the plane of polarization of the light incident on it.
Hence, P3 must be rotated through (90° + 30°) = 120°.
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.