Question

A slit of width a is illuminated by red light (l = 6500 Å). The first minima for red light will fall at q = 30º, when a will be –

• A. 3250 Å
• B. 6.5 × 10^–4 cm
• C. 1.3 micron
• D. 2.6 × 10^–4 cm

Answer 1

Answer: (C)

1.3 micron

Answer 2

sinq = $\frac{n\lambda}{a}$, for n = 1, sinq = $\frac{\lambda}{a}$

\ a = $\frac{\lambda}{\sin\theta} = \frac{6500 \times 10^{- 10}}{\sin 30º}$

a = 13000 × 10^–10 m

a = 1.3 × 10^–6 m

a = 1.3 micron