In: Physics
A uniform thin film of alcohol (n=1.36) lies on a flat glass plate (n=1.56). When monochromatic light, whose wavelength can be changed, is incident normally, the reflected light is a minimum for ?=515nm and a maximum for ?=644nm.
What is the minimum thickness of the film?
tmin = .....?..... nm
When the incident llight reflects from top of the alcohol has a phase change of 1 =
The wave reflects from the glass at the bottom of the alcohol has a phase change is
2 = (2t/film) *2 +
For constructive interference the net phase change is
= 2 - 1 = m1 *2 , m1 = 1,2.3.....
then t =1/2*film *m1 = 1/2*(1/nfilm)*m1
For destructive interference the net phase change is
= 2 - 1 = 1/4*film *(2*m2+1) = 1/2*(2/nfilm)*(2*m2+1)
from the two equations
1/2*(1/nfilm)*m1 = 1/2*(2/nfilm)*(2*m2+1)
(2m2+1)/m1 = 1 /2 = 644/515 = 1.25
Thus we have
m1 = m2 = 2
then thickness of the film
t = 1/2*(644/1.36)*2
t = 473.52 nm