A-LevelPhysicsSuperposition9702/22 · May/June 2017 · Q610 Marks

the part played by diffraction in the production of the fringes,

(a)(i)the part played by diffraction in the production of the fringes,[2]
(a)(ii)the reason why a double slit is used rather than two separate sources of light.[1]
(b)A laser emitting light of a single wavelength is used to illuminate slits S₁ and S₂, as shown in Fig. 6.1. [Figure 6.1] An interference pattern is observed on the screen AB. The separation of the slits is 0.48mm. The slits are 2.4m from AB. The distance on the screen across 16 fringes is 36 mm, as illustrated in Fig. 6.2. [Figure 6.2] Calculate the wavelength of the light emitted by the laser.[3]
(c)(i)State and explain what is observed at P.[2]
(c)(ii) 1.the phase difference between the waves at D₁ and at D₂ is 180°,[1]
(c)(ii) 2.the intensity of the wave from D₁ is less than the intensity of that from D₂.[1]

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Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

[B1] waves at (each) slit/aperture spread
[B1] (into the geometric shadow) wave(s) overlap/superpose/sum/meet/intersect
[B1] there is not a constant phase difference/coherence (for two separate light source(s))
[C1] x = λD/a
[C1] λ = (36 × 10⁻³ × 0.48 × 10⁻³) / (16 × 2.4)
[A1] = 4.5 x 10⁻⁷ m
[B1] no movement of the water/water is flat/no ripples/disturbance
[B1] the path difference is 2.5λ or the phase difference is 900° or 5π rad
[B1] surface/water/P vibrates/ripples and as (waves from the two dippers) arrive in phase
[B1] surface/water/P vibrates/ripples and as amplitudes/displacements are no longer equal/do not cancel

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