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# Light of wavelength $\lambda$ from a point source falls on a small circular obstacle of diameter 'd' . dark and bright circular rings around the central bright spot are formed on a screen beyound the obstacle. The distance between the screen and obstacle is D. Then, the condition for the formation of rings is

$\begin {array} {1 1} (1)\;\sqrt{ \lambda} \approx \large\frac{d}{4D} & \quad (2)\;\lambda \approx \large\frac{d^2}{4D} \\ (3)\;d \approx \large\frac{\lambda^2}{D} & \quad (4)\;\lambda \approx \large\frac{D}{4} \end {array}$

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