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Question:

In a diffraction pattern due to a single slit of width 'a', the first minimum is observed at an angle 30° when light of wavelength 5000Å is incident on the slit. The first secondary maximum is observed at an angle of :

sin⁻¹(12)

sin⁻¹(14)

sin⁻¹(23)

sin⁻¹(34)

Solution:

For first minima: sin 30° = λ/a = 1/2
First secondary maxima will be at: sin θ = 3λ/2a = 3/2 × 1/2 ⇒ θ = sin⁻¹(3/4)