Question

A slit of width d is illuminated by white light. The first minimum for red light ($\lambda = 6500Å$)will fall at $\theta = 30^{0}$when d will be

• A. 3200Å
• B. 6.5×10^-4
• C. 1.3
• D. 2.6×10^-4

Answer 1

Answer: (C)

1.3

Answer 2

For first minimum $a\sin\theta = 1\lambda$

$a = \frac{\lambda}{\sin\theta} = \frac{6.5 \times 10^{- 7}}{\sin 30º} = 13 \times 10^{- 7} = 1.3micron$