Question

$\text{Assertion:}$ In the figure shown, zero order maxima will lie above point $O$.
$\text{Reason:}$ Zero order maxima normally means a point where path difference is zero.

• A. Both the Assertion and Reason are true, and the Reason is the correct explanation of the Assertion.
• B. Both the Assertion and Reason are true, but Reason is not the correct explanation of Assertion.
• C. The Assertion is true, but the Reason is false.
• D. The Assertion is false, but the Reason is true.

Answer 1

The correct option is B Both the Assertion and Reason are true, but Reason is not the correct explanation of Assertion.


A point on the screen will be a bright fringe (maximum) when constructive interference takes place.

The condition for constructive interference is,

$\Delta \varphi =n\left(2\pi \right)$ and $\Delta x=n\lambda$

Zero order maxima or central maxima corresponds to the point where path difference between waves is zero.

Here, $n=0$ for zero order maxima.

Since, the glass slab is introduced in front of the upper slit, the ray through $S_1$ will cover a longer optical path.

$\Rightarrow$ Thus in order to obtain central maxima $(\Delta x=0)$, the light wave through $S_2$ will have to cover more distance to nullify the path difference.

$\therefore$ Fringe pattern will get shifted upwards.

The shift due to the optical path of glass plate.

Hence, option $\left(B\right)$ is correct.