Question
Monochromatic electromagnetic radiation from a distant source passes through a slit. The diffraction pattern is observed on a screen 2.50 m from the slit. If the width of the central maximum is 6.00 mm, what is the slit width if the wavelength is
(a) 500 nm (visible light);
(b) 50 µm (infrared radiation);
(c) 0.500 nm (X-rays)?

Answer

Data:2W = 6mm ∴ W= 3 mm = 3 × 10-3 m, y = 2.5 m,

(a) $\lambda_1=500 \mathrm{~nm}=5 \times 10^{-7} \mathrm{~m}$

(b) $\lambda_2=50 \mu \mathrm{m}=5 \times 10^{-5} \mathrm{~m}$

(c) $\lambda_3=0.500 \mathrm{~nm}=5 \times 10^{-10} \mathrm{~m}$

Let a be the slit width.

(a) $\begin{aligned} W & =\frac{y \lambda_1}{a} \\ \therefore a & =\frac{y \lambda_1}{W}=\frac{(2.5)\left(5 \times 10^{-7}\right)}{3 \times 10^{-3}} \\ & =4.167 \times 10^{-4} m \\ & =0.4167 mm \end{aligned}$

(b) $\begin{aligned} W & =\frac{y \lambda_2}{a} \\ \therefore a & =\frac{y \lambda_2}{W}=\frac{(2.5)\left(5 \times 10^{-5}\right)}{3 \times 10^{-3}} \\ & =4.167 \times 10^{-2} m \\ & =41.67 mm \end{aligned}$

(c) $\begin{aligned} W & =\frac{y \lambda_3}{a} \\ \therefore a & =\frac{y \lambda_3}{W}=\frac{(2.5)\left(5 \times 10^{-10}\right)}{3 \times 10^{-3}} \\ & =4.167 \times 10^{-7} m \\ & =4.167 \times 10^{-4} mm \end{aligned}$

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