Question

In Fresnel diffraction, if the distance between the disc and the screen is decreased, the intensity of central bright spot will

• A. increase
• B. decrease
• C. remain constant
• D. none of these

Answer 1

The correct option is B decrease

$A=n\pi d\lambda \Rightarrow nd=\frac{A}{n\lambda }=constant$

$\Rightarrow n\propto \frac{1}{d}(n=$ number of blocked HPZ$)$ on decreasing $d,n$ increases, hence intensity decreases.