Polarization of Light

1. Polarization of Light

2. Demo: Two polarized lens, overhead, cellophane tape placed on a piece of plastic or/and wrapped around a clear ruler. Place one polarized lens on overhead. Describe what you see. Place second polarized lens on top of first and rotate. Describe what you see. Place the plastic sheet with cellophane tape on it in between the two Polaroid lens. Rotate one lens. Describe what you see. Place the ruler with cellophane tape on it in between the two Polaroid lens. Rotate one lens. Describe what you see.

3. A light bulb emits visible light of various frequencies (from red to violet) and is not polarized. If you could see the light from an “end” view, the different light waves would be vibrating up and down in all directions; ie. They are not polarized!

4. A polarized lens is consists of many long chain molecules all orientated in the same direction. These molecules are too small to see with the naked eye but they are there. This light wave will be able to pass through this opening. The actual lens would have thousands of these molecular lines. This light wave will NOT be able to pass through this opening.

5. Waves that not are orientated vertically or horizontally would still have a component of the wave that is. Waveo Wavey ø The light intensity as it passes through a polarized lens is a function of its cosine angle. Wavex I2 = I1 (cos ø)2 (After 1st lens) (Known as Malus’ Law) (After 2st lens and so on) I3 = I2 (cos ø)2 Now if you have been following along you should b e able to understand how a polarizing lens works and to explain the demo you saw earlier. Each lens blocks about ½ of the light waves When one Polaroid lens is placed on top of another in the correct orientation - no light passes through.

6. From a side view: No light Unpolarized light (Intensity Io) polarized light (Intensity 1/2Io) Remember when I placed the tape & plastic in between the two polaroid’s. Why was light able to now pass through? The tape and plastic changes the orientation of some of the light waves. some light Unpolarized light (Intensity Io) Tape covered plastic sheet

7. Polarization by Reflection Light can also be polarized by light from reflecting of a nonmetallic surface (like a pond or lake) at any angle other than perpendicular. After the reflection the light is partially polarized in horizontal direction. Unpolarized light Without polarized lens With polarized lens

8. Another Example The amount of polarization in the reflected beam depends on the angle, varying from no polarization at normal incidence (90o) to 100% polarization at an angle known as the polarizing angle øp. The angle is related to the index of refraction of the two materials on either side of the boundary of the reflecting substance. n2 Tan øp = n1 Where: n1 = index of refraction of the material in which the beam is travelling n2 = index of refraction of the material beyond the reflecting boundary

9. If light is travelling through air, then n1 = 1.00 and the equation becomes: Tan øp = n (This know as Brewster’s Law) The angle øp is know as Brewster’s angle Reflected light ray Incident light ray øp øp At Brewster’s angle, the addition of the refracted angle and Brewster’s angle add up to 90o Air water 90o øp+ ør = 90o ør Refracted light ray

10. Advertisement for Polaroid sunglasses. Now you know how they work. Would you pay more for them over regular sunglasses? Read bout Liquid Crystal Displays on p. 688 -89 of your text book. Important in understanding how your calculators and LCD TV’s work!