Question

Assertion : When a thin transparent sheet is placed in front of both the slits of Young’s experiment, the fringe width will remains same.
Reason : In Young’s experiment, the fringe width is directly proportional to wavelength of the source used.

• A. If both assertion and reason are true and reason is the correct explanation of assertion
• B. If both assertion and reason are true but reason is not the correct explanation of assertion
• C. If assertion is true but reason is false
• D. If both assertion and reason are false

Answer 1

Answer: (B)

Fringe width $\beta = \lambda D/d$ shall remain the same as the waves travel in air only, after passing through the thin transparent sheet. Due to introduction of thin sheet, only path difference of the wave is changed due to which there is shift of position of fringes only, which is given as
$\Delta x = \frac{D(\mu -1)t}{d}$, where $\mu$ is refractive index of thin sheet and $t$ is its thickness.