Question

When you spread oil (noil = 1.50) on water (nwater = 1.33) and have white light incident from above, you might observe different reflected colors that depend on the thickness of the oil on the water at each point. Suppose we find a location where we see green light reflected from the thin film of oil. If we slowly increase the thickness of oil at that location, what happens to the observed reflected color? Show all equations and calculations

Answer 1

at first interface,

the incident light is in air and has less refractive index than thin film.

for construcitve interference, condition is

thickness t = (m +1/2) L0/2n , where m = 0,1,2,3,.......

Lo is the wavelength

Condition for Destrutive interference is

t = mL0/2n where m = 0, 1,2,3,4,......

the minimum thickness of the film for the reflected rays to interfere construtively is

t = (m +1/2) L0/2n for Lo = 750 nm and n =1.5 m = 0

t = 125 nm is the minimum thickness of the oil that would allow us to see the red light

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now for Lo = 380 nm, n = 1.5 and m = 0

t = (0+1/2) * 380 nm/(2*1.5)

t = 63.3 nm is the minimum thickness of the oil that would allow us to see the violet light