A linear aperture whose width is 0.02 cm is placed immediately in front of a lens of focal length 60 cm. The aperture is illuminated normally by a parallel beam of wavelength 5 × 10⁻⁵ cm. The distance of first dark band from centre of diffraction pattern is:
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Question

A linear aperture whose width is 0.02 cm is placed immediately in front of a lens of focal length 60 cm. The aperture is illuminated normally by a parallel beam of wavelength 5 × 10⁻⁵ cm. The distance of first dark band from centre of diffraction pattern is:

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G1 Physics · Accepted Answer

Step 1: First diffraction minimum of a slit satisfies sin θ = λ/a. Step 2: For small angle, y ≈ fθ = fλ/a. Step 3: y = 60 × (5 × 10⁻⁵)/(0.02) cm = 0.15 cm. Final answer: (D) 0.15 cm.