PHYSICS 305 GENERAL PHYSICS 3 LAB
INSTRUCTOR: Dr. M. Azad Islam
POLARIZATION OF LIGHT
1. Purpose: To investigate the linear polarization of light. To measure the intensity of light as a function of polarization angle. To verify the Malus law of polarization.
2. Equipment: Optical bench, photometer, fiber-optic cable & its mount, tungsten filament lamp, convex lens (f=127mm), 2 magnetic mounts, iris, 3 polarizers.
CAUTION: Please handle all the equipment with care. DO NOT touch any optical element on the face, or try to wipe it clean. Hold the optical component by the sides at all times.
3. Introduction: Classical electromagnetic theory first worked out by Sir James Clerk Maxwell showed visible light to be a part of the electromagnetic waves. Light like any other electromagnetic waves such as radiowaves or microwaves, is composed of vibrating electric and magnetic fields at right angles to each other in isotropic medium, while its direction of propagation is normal to both the fields at all times. An expression with time and space dependent electric field is sufficient to represent light or any other electromagnetic wave. It is customary to define the direction of linear polarization of light, to be the direction of electric field. A linearly polarized light, sometimes also referred to as plane polarized light, has its electric field vector oriented along a constant direction. Beside linear polarization, light can also be circularly and elliptically polarized; in which electric field vector rotates in a helical path as it moves forward. One talks about left or right circularly polarized light, depending upon the sense of rotation of the electric field vector, either clockwise or counterclockwise. The most general state of polarization of light is called elliptical polarization. Light from ordinary lamp is randomly polarized. Linear polarizers such as Polaroid plates are used for producing linearly polarized light. Each polarizer has a unique direction called axis of transmission (or passing axis) which defines the direction of electric field, once light passes through such a polarizer. Light will preserve its sense of polarization unless disturbed in its path.
4. Theory: Malus law gives the intensity dependence of light when it passes through a pair of linear polarizers.
I = Iocos2q
Where q is the angle between the axes of polarization of the two polarizers and Io is the intensity of light when the two axes are parallel, that is q = 0o.
5. Experiment: Use the optical bench to mount the lamp at one end and the detector on the other end. An iris changes the width of the beam. The lens is adjusted to focus the image of the hot filament on the fiber-optic cable for maximum light intensity. The first polarizer remains fixed while the second is rotated to obtain intensities for various angles of q. Before taking data, show your experimental set up to your instructor.
6. Data Collection: Align the axes of the two polarizers to measure the maximum intensity Io for q =0o. Measure intensity of light for various angles of q. Change the angle from zero to 360o in steps of 10o. When two polarizers are crossed for minimum light, place a 3rd polarizer between the two, rotate this polarizer and observe the light intensity on the photometer for various angles and, record its value of light output and the angle for maximum light output.
7. Plot two graphs on the same page, one for the relative intensity I/Io versus q (experiment) and the other for cos2q versus q (theory). Explain the two graphs in terms of your expectations.
8. Sketch the electric field vector in the light wave at locations near the lamp and after each of the two polarizers before detection.
9. Derive Malus law from your knowledge of the electric field as a wave.
10. Explain your observation when the 3rd polarizer was introduced between two other crossed.
Figure: TO SHOW EXPERIMENTAL ARRANGEMENT
Updated 08/30/2000