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Concepts Of Physics MCQ Edition [Volume 1]
Exam
Concepts Of Physics MCQ Edition [Volume 1]
Subject
Physics
Chapter
Light Waves

Concepts Of Physics MCQ Edition [Volume 1] · Physics · Light Waves

Light of wavelength 560 nm passes through a pinhole of diameter...

Light of wavelength 560 nm560 \text{ nm} passes through a pinhole of diameter 0.20 mm0.20 \text{ mm} and strikes a wall located at a distance of 2.00 m2.00 \text{ m}. Determine the diameter of the central bright spot formed on the wall.

Options

  1. A0.68 cm0.68 \text{ cm}
  2. B1.12 cm1.12 \text{ cm}
  3. C0.56 cm0.56 \text{ cm}
  4. D1.37 cm1.37 \text{ cm}

Correct answer

D. 1.37 cm1.37 \text{ cm}

Step-by-step solution

The angular radius of the central bright spot (Airy disk) formed by diffraction through a circular aperture (pinhole) is given by:

θ=1.22λd\theta = \dfrac{1.22 \lambda}{d}

where λ\lambda is the wavelength of light and dd is the diameter of the pinhole.

The linear radius of the central bright spot on a screen at a distance DD is:

R=Dθ=1.22λDdR = D\theta = \dfrac{1.22 \lambda D}{d}

The diameter of the central bright spot is:

Dspot=2R=2.44λDdD_{\text{spot}} = 2R = \dfrac{2.44 \lambda D}{d}

Given values:
λ=560 nm=560×10−9 m\lambda = 560 \text{ nm} = 560 \times 10^{-9} \text{ m}
d=0.20 mm=0.20×10−3 md = 0.20 \text{ mm} = 0.20 \times 10^{-3} \text{ m}
D=2.00 mD = 2.00 \text{ m}

Substituting the values into the formula:

Dspot=2.44×560×10−9×2.000.20×10−3D_{\text{spot}} = \dfrac{2.44 \times 560 \times 10^{-9} \times 2.00}{0.20 \times 10^{-3}}

Dspot=2.44×560×10−6×10D_{\text{spot}} = 2.44 \times 560 \times 10^{-6} \times 10

Dspot=2.44×5.6×10−3 mD_{\text{spot}} = 2.44 \times 5.6 \times 10^{-3} \text{ m}

Dspot=13.664×10−3 m=1.3664 cmD_{\text{spot}} = 13.664 \times 10^{-3} \text{ m} = 1.3664 \text{ cm}

Rounding to two decimal places, we get 1.37 cm1.37 \text{ cm}.
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