Question

Q17/40
A thin layer of colorless oil is spread uves
water in a container (u = 14). If the light
of wavelength 640 nm is absent in the
reflected light, what is the minimum
thickness of oil layer?
OPTIONS
179.6 nm
98.3 nm
207.6 nm
214.3​

Answer 1

Given:

Refractive Index = 1.4

Wavelength 640 nm

Find: We have to calculate the thickness of oil layer

Step by Step Solution:

Let t be the thickness of the oil layer spread on water.

We have the expression for minimum thickness,

$2 \mu t cosr = n\lambda$

for minimum thickness we take n = 1,

$2\times 1.4\times t = 1 \times 640\\\\2.8 \times t = 640\\\\t = \dfrac{640}{2.8}\\\\t = 228.57nm$

In this question there is a wrong option.