Question

Light is reflected normally from thin layer of oil over water. Refractive index of oil is 1.2 and that of water is $\frac{4}{3}$. Layer of oil looks shining orange colour $(\lambda =600nm)$ if thickness of oil layer is

• A. $1\mu m$
• B. $0.75\mu m$
• C. $0.125\mu m$
• D. $0.625\mu m$

Answer 1

Answer: (A), (B)

The correct options are
A $1\mu m$
B $0.75\mu m$

For constructive interference of reflected light,

$2\mu t=n\lambda$

$t=\frac{n\lambda }{2\mu }=\frac{n600nm}{2\times 1.2}=n\frac{0.6}{2.4}\mu m$

n = 1, 2, 3, .......