I am looking at a film of oil n 12 on water n 133 When I shi

I am looking at a film of oil (n= 1.2) on water (n= 1.33). When I shine green light = 550 nm, on it, it looks dark (no reflection). (Assume the water is very deep, i.e. there is no reflection off the bottom of the water).

a) What is the minimum thickness of the film?

b) Given the thickness in (a) above, what is the next shorter wavelength for which the film would appear bright (strong reflection)?

Solution

for no reflection there should be a minimum path difference of /2

for strong reflection path difference should be n  where n is integer

note: using the thin film u are creating a path difference which is proportional to the thickness of the film.also when light move from rsrer to denser medium there is a  path diff of /2 ...i hope it helped u :)

(a) 2nt = /2
so t =/(4n) = 550 nm/(4*1.2) = 115 nm
(b) 2nt == 275 nm   (strong reflection)