A-LevelPhysicsQuantum physics9702/41 · Oct/Nov 2020 · Q119 Marks

A photon of wavelength 540nm collides with an isolated stationary electron, as illustrated in Fig. 11.1. [Figure 11.1]. The photon is deflected elastically by the electron. The wavelength of the deflected photon is 544 nm.

(a)(i)State what is meant by a photon.[2]
(a)(ii)On Fig. 11.1, draw an arrow to indicate the approximate direction of motion of the deflected electron.[1]
(b)(i)Calculate: the momentum of the deflected photon[2]
(b)(ii)the energy transferred to the deflected electron.[2]
(c)Another photon of wavelength 540nm collides with an isolated stationary electron. Explain why it is not possible for the deflected photon to have a wavelength less than 540 nm.[2]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2020). The mark or mark type is shown in brackets.

[M1] quantum of energy
[A1] of electromagnetic radiation
[B1] arrow (on Fig. 11.1) pointing upwards and to the right
[C1] λ = h/p
[C1] p = (6.63 × 10⁻³⁴) / (544 × 10⁻⁹)
[A1] = 1.22 x 10⁻²⁷ N s
[C1] energy = hc / λ
[C1] = 6.63 × 10⁻³⁴ × 3.00 × 10⁸ × (540⁻¹ – 544⁻¹) × 10⁹
[A1] = 2.7 × 10⁻²¹ J
[B1] (smaller wavelength corresponds to) greater photon energy
[B1] any one point from: (deflected) photon loses energy (so not possible) / (deflected) photon would need to gain energy (so not possible) / electron would need to lose energy (so not possible) / initially electron energy is zero (so not possible)

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