Light of wave–length $ \lambda $ is incident on a slit of width d. The resulting diffraction pattern is observed on a screen placed at a distance D. The linear width of the principal maximum is equal to the width of the slit, then D = ______.
The linear width of the principal maximum in a single-slit diffraction pattern is given by $2\lambda D / d$. If this width is equal to the width of the slit (d), then we can set up the equation $2\lambda D / d = d$. Solving for D, we get $D = \frac{d^2}{2\lambda}$. Hence, the correct option is $\frac{d^2}{2\lambda}$.
