Question

The minimum thickness of film which will strongly reflect the normally incident light of $\lambda =600nm$, the R.I. of material of film is $1.25$:

• A. $120nm$
• B. $100nm$
• C. $110nm$
• D. $90nm$

Answer 1

The correct option is A $120nm$

For Strong reflection The optical density should be at least $\frac{\lambda }{2}$

Let

$\lambda \to$ wave length

$t\to$ thickness

$\mu \to$ R>I of material

For strong reflection

$\therefore 2\mu t=\frac{\lambda }{2}$

$\therefore t=\frac{\lambda }{4\mu }$

$=\frac{600}{4\times 1.25}$

$\therefore t=120nm$

$\therefore$ Minimum thickness should be $120nm$