-Thin Film Interference -Polarization Physics Mrs. Coyle

-Thin Film Interference -Polarization

PhysicsMrs. Coyle

Thin Film Interference

http://graphics.stanford.edu/courses/cs348b-competition/cs348b-08/finals/soap.png

Thin Film Interference

Thin Film Interference

http://laser.physics.sunysb.edu/~hilary/graphics/interference.gif

Thin Film Interference

Thin Film Interference

Ray 1 travels into the film and back it travels 2t (twice the thickness of the film).

The path difference between the rays when they come out of the film is 2t.

Condition for Constructive Interference:

The reflected rays coming out of the film (5) and the rays that reflected on the film interface with the air(2), must have a path difference of an integer multiple of /2.

This is because (2) flipped going from a less dense to a more dense medium.

Note: is the wavelength in the film.

Condition for Constructive Interference

Path Difference= 2t =m /2

so thickness, t= m /4 where m=1,3, 5… t is the thickness of the film.

Minimum thickness occurs when m=1.

Problem

Calculate the minimum thickness of a soap bubble that results in constructive interference when a light of wavelength 500nm shines on it.

Answer: 125nm

Newton’s RingsWaves reflecting from two surfaces can interfere.Here light is reflected at glass/air and air/glass interfaces

(sandwiched).

The interference produces concentric rings of rainbow colors.

Uses• To get rid of unwanted interference

on camera lenses etc use appropriate coatings. Here, coatings get rid of reflecting light of middle of spectrum so the lens appears red/violet.