Question

A thin film of oil of thickness t is floating on water. The oil has index of refraction no = 1.4. There is air above the oil. When viewed from the air in the direction normal to the surface, there is constructive interference of reflected red light with l = 675 nm. Find the minimum thickness t of the oil film that will give constructive interference of reflected light with l = 675 nm.

Answer 1

Answer:

t = 120.5 nm

Explanation:

given,    

refractive index of the oil = 1.4

wavelength of the red light = 675 nm

minimum thickness of film = ?

formula used for the constructive interference

$2 n t = (m+\dfrac{1}{2})\lambda$

where n is the refractive index of oil

t is thickness of film

for minimum thickness

m = 0

$2 \times 1.4 \times t = (0+\dfrac{1}{2})\times 675$

$t = \dfrac{0.5\times 675}{2\times 1.4}$

        t = 120.5 nm

hence, the thickness of the oil is t = 120.5 nm