A-LevelPhysicsSuperposition9702/23 · Oct/Nov 2019 · Q55 Marks

Light waves emerging from the slits of a diffraction grating are coherent and produce an interference pattern. A narrow beam of light from a laser is incident normally on a diffraction grating, as shown in Fig. 5.1. [Figure 5.1] Spots of light are seen on a screen positioned parallel to the grating. The angle corresponding to each of the second order maxima is 51°. The number of lines per unit length on the diffraction grating is 6.7 x 10⁵m⁻¹.

(a)(i)coherence[1]
(a)(ii)interference.[1]
(b)(i)Determine the wavelength of the light.[2]
(b)(ii)State and explain the change, if any, to the distance between the second order maximum spots on the screen when the light from the laser is replaced by light of a shorter wavelength.[1]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2019). The mark or mark type is shown in brackets.

[1] (coherence means) constant phase difference (between waves)
[1] (interference is) the sum/addition/combination of the displacements of overlapping/meeting waves
[1] nλ = d sinθ
[1] λ = sin 51° / (2 × 6.7 × 105) = 5.8 x 10-7 m
[1] smaller angle (corresponding to second order maxima and so) shorter distance (between second order maxima spots)

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