Question

A slit of width a is illuminated by white light. For red light (l = 6500Å), the first minima is obtained at q = 30º. Then the value of a will be –

• A. 3250 Å
• B. 6.5 × 10^–4 mm
• C. 1.25 microns
• D. 2.6 × 10^–4 cm

Answer 1

Answer: (C)

1.25 microns

Answer 2

For 1^st minima q = $\frac{\lambda}{d}$ or d = $\frac{\lambda}{\theta}$

d = $\frac{6500 \times 10^{–8} \times 6}{\pi}$ $\left( \because 30º = \frac{\pi}{6} \right)$

= 1.24 × 10^–4 cm = 1.24 micron