A-LevelPhysicsWaves9702/21 · May/June 2021 · Q410 Marks

For a progressive wave, state what is meant by wavelength.

(a)For a progressive wave, state what is meant by wavelength.[1]
(b)A light wave from a laser has a wavelength of 460 nm in a vacuum. Calculate the period of the wave.[3]
(c)The light from the laser is incident normally on a diffraction grating. Describe the diffraction of the light waves at the grating.[2]
(d)(i)The gradient of the graph is G. Determine an expression, in terms of G, for the distance d between the centres of two adjacent slits in the diffraction grating.[2]
(d)(ii)On Fig. 4.1, sketch a graph to show the results that would be obtained for the second-order maxima. [Figure 4.1][2]

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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] distance moved by wavefront/energy during one cycle/oscillation/period (of source) or minimum distance between two wavefronts or distance between two adjacent wavefronts
[C1] v = λ/T or v = fλ and f = 1/T
[C1] T = 460 × 10⁻⁹ / 3.00 × 10⁸
[A1] = 1.5 × 10⁻¹⁵ s
[B1] waves pass through/enter the slit(s)
[B1] waves spread (into geometric shadow)
[C1] nλ = d sinθ
[C1] G = sinθ/λ
[A1] d = 4/G
[M1] straight line from 400 nm to 700 nm that is always below printed line
[A1] straight line has smaller gradient than printed line and is 5 small squares high at wavelength of 700 nm

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