A-LevelPhysicsSuperposition9702/23 · May/June 2021 · Q48 Marks

State the principle of superposition.

(a)State the principle of superposition.[2]
(b)Two waves, with intensities I and 4I, superpose. The waves have the same frequency. Determine, in terms of I, the maximum possible intensity of the resulting wave.[2]
(c)Coherent light of wavelength 550nm is incident normally on a double slit of slit separation 0.35 mm. A series of bright and dark fringes forms on a screen placed a distance of 1.2m from the double slit, as shown in Fig. 4.1. The screen is parallel to the double slit. [Figure 4.1]
(c)(i)Determine the distance between the centres of adjacent bright fringes on the screen. distance = ............................................................ m[3]
(c)(ii)The light of wavelength 550nm is replaced with red light of a single frequency. State and explain the change, if any, in the distance between the centres of adjacent bright fringes.[1]

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Mark scheme

Marking points from the official mark scheme (May/June 2021). The mark or mark type is shown in brackets.

[B1] (when two or more) waves meet/overlap (at a point)
[B1] (resultant) displacement is sum of the individual displacements
[C1] intensity ∝ amplitude²
[A1] maximum intensity = 9I
[C1] x = λD/a
[C1] = (550 × 10⁻⁹ × 1.2) / (0.35 × 10⁻³)
[A1] = 1.9 x 10⁻³ m
[B1] red light has longer wavelength (than 550 nm) so distance (between fringes) increases

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