Scattering

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

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

Ray Optics vs. Scattering

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

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

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

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

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

Sunset Do demo

Polarization Light as an electromagnetic wave

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

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

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

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

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

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

Polarizing Filters

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

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

Wire-Grid Polarizers

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