1. Scattering

2. Scattering and Polarization
Light Scattering
Blue skies
Red sunsets
Polarization
Review EM waves
Polarizing filters

3. Light Scattering We have been treating light as rays: straight lines
If you zoom way in, recall that light is really a WAVE
Ray optics don’t work anymore, and we consider something called scattering
Scattering is the reason the sky is blue, sunsets are red, why you can see a laser beam in the air

4. Ray Optics vs. Scattering

5. Rayleigh Scattering
The simplest type of scattering is called “Rayleigh scattering”
The rule you need to remember about Rayleigh scattering is that:
The shorter the wavelength of the incident light, the more light is scattered
In other words,
Blue light scatters more than red light

6. Rayleigh Scattering: Blue Sky
Blue light scatters more than red light
This is the reason that the sky is blue
The light from the sun contains all visible wavelengths
It scatters from particles in the atmosphere
Blue light scatters more than red, so we see predominantly blue light when we look at the sky

7. Blue Sky
Note that the sun sends rays in all directions as well

8. Concept Question Black
We see the sky as blue because of sunlight scattering off of particles in the atmosphere. The moon has no atmosphere. Standing on the moon, what color does the sky look when the sun is shining? (during the moon’s “daytime”)
Blue
Red
Black
Some other color
Black

9. Rayleigh Scattering: Sunsets
Blue light scatters more than red light
This is also the reason that sunsets are red
At sunset, light from the sun has to travel through more of the atmosphere, and the blue light scatters away before the light reaches your eyes
One way to think about it is that your sunset is blue sky somewhere else on the earth, so they are getting the blue light, and only the red and orange get to you

10. Sunset
Do demo

11. Polarization
Light as an electromagnetic wave

12. Electromagnetic Waves
The polarization is defined as the direction of oscillation of the electric field
Polarized “along the z-axis”

13. Along which axis is this light wave polarized?Y Z x
Electric field: pink
Magnetic field: blue y X z

14. Polarized Light Waves
Light waves are considered polarized if their electric fields are all oriented in the same direction x y x y
Linearly polarized in
the y-direction
Linearly polarized in
the x-direction

15. Polarized Light Waves
The electric field can be aligned in any direction in the xy-plane
We can describe this polarization as being composed of an x-component and a y-component x y x y

16. Un-polarized light
Light waves are un-polarized if their electric fields are rapidly and randomly varying

17. Polarizing Filters
A polarizing filter (used in sunglasses, camera filters, 3D movies, etc) is a device that transmits light with one polarization only, and rejects all other polarizations
There are two main ways to do this:
Absorb the incorrectly polarized light
Reflect the incorrectly polarized light

18. Polarizing Filters

19. Absorptive Polarizing Filters
Also know as “Polaroid” film, after the company that manufactures it.
Made by processing plastic films containing special crystals
Inexpensive, used in sunglasses and camera filters
Because they absorb the “rejected” light, they are not suitable for high power applications, as they will absorb the light energy and melt or deform

20. Reflective Polarizing Filters
Also called “wire-grid” polarizers
More expensive to manufacture
Can be used with high incident power
Used primarily for laser applications

21. Wire-Grid Polarizers

22. Thanks…