Question

A slit of width a is illuminated by white light. For red light $ (\lambda= 6200 \,\mathring{A} $ ), the first minima is obtained at a diffraction angle of $ 30^{\circ} $ . Then the value of $ a $ is

• A. $ 3250 \,\mathring{A} $
• B. $ 6.5\times 10^{-4}\,mm $
• C. $ 1.24 $ micron
• D. $ 2.6\times 10^{-4}\,cm $

Answer 1

Answer: (C)

For first minima
$a \sin \theta_{1}=\lambda$
$\therefore a=\frac{\lambda}{\sin \theta_{1}}=\frac{6200 \times 10^{-10}}{\sin 30^{\circ}}$
$=1.24 \times 10^{-6}\, m$
$=1.24\, \mu$