Question

In the ideal double slit experiment, when a glass plate of refractive index 1.5 and thickness t is introduced in the path of one of the interfering beams of wavelength l, the intensity at the position of central maximum remains unchanged. Minimum thickness of glass plate is

• A. 2l
• B. 2l/3
• C. l/3
• D. l

Answer 1

Answer: (A)

2l

Answer 2

The path difference due to slab should be integral multiple of l,

\ Dx = (µ _ 1)t = nl n = 1, 2, 3, …..

t =$\frac{n\lambda}{(\mu - 1)}$

for t to be minimum, n = 1

̃ t =$\frac{1 \times \lambda}{(1.5 - 1)}$= 2l

Therefore the answer is (1)