learning objective 9

1. Thin Film Interference Problem Dissection
By Andrew Welsh
How this works: use a piece of paper to cover up with step below the
one you are working and check your work by moving the paper down a
step.
Problem:
If n1 is air and has a value of 1, n2
is oil and has a value of 1.4 and n3
is water and has a value of 1.33
how thick is the oil layer if it
appears to be yellow (lambda =
500 nm)?
Solution by steps:
1. determine if there is a phase
shift on each boundary. A phaseshift occurs when there is the medium
the light is traveling through enters a medium with a higher n.
Is n2 higher than n1? ______
Is n3 higher than n2? ______
2. (yes,no)Theextra distance traveled is equal to 2d. This works wellwith
the equation of the firstbeam being wavelength/2 as if the oil had a
depth of 0 the 2 would cancel out and the extra distance traveled would
simple equal the wavelength.
3. Use the equation ½*wavelength/n2 to solve for 2d or the extra distance
traveled. This can be rewritten as d=1/4*wavelength/n2
4. Plugging this in you get 500*10^-9m/(4*1.4)=89*10^-9mor 89 nm.