Question

A ray of light from a monochromatic point source of light is incident at a point on the screen. If a thin mica film of thickness ' $t$ ' and refractive index ' $n$ ' is introduced in its path, then the optical path}

• A. is decreased by $(n - 1)t$.
• B. is increased by $(n + 1)t$.
• C. is not affected.
• D. is increased by $(n - 1)t$.

Hint:

Introducing a medium of higher refractive index always increases the optical path.

Answer 1

The Correct Option is D

Step 1: Definition
Optical path $= \mu \times \text{physical path}$.
Step 2: Comparison
Without film, path length $= t$.
With film, optical path $= n \times t$.
Step 3: Difference
Increase $= nt - t = (n - 1)t$.
Final Answer: (D)