Question

Light of wavelength $\lambda$ from a point source falls on a small circular obstacle of diameter ${}^{\prime }{d}^{\prime }$. Dark and bright circular rings around a central bright spot are formed on a screen beyond the obstacle. The distance between the screen and obstacle is $D$. Then, the condition for the formation of rings is:

• A. $\lambda \approx \frac{d^2}{4D}$
• B. $d\approx \frac{{\lambda }^2}{D}$
• C. $\lambda \approx \frac{D}{4}$
• D. $\sqrt{\lambda }=\frac{d}{4D}$

Answer 1

The correct option is A $\lambda \approx \frac{d^2}{4D}$

Rings are formed when constructive interference occurs between the light waves from two slits. Condition for this to happen is:

$\lambda \approx \frac{d^2}{4D}$