A-LevelPhysicsQuantum physics9702/41 · May/June 2020 · Q109 Marks

(a) White light passes through a cloud of cool low-pressure gas, as illustrated in Fig. 10.1. [Figure 10.1] For light that has passed through the gas, its continuous spectrum is seen to contain a number of darker lines. Use the concept of discrete electron energy levels to explain the existence of these darker lines.

(a)White light passes through a cloud of cool low-pressure gas, as illustrated in Fig. 10.1. [Figure 10.1] For light that has passed through the gas, its continuous spectrum is seen to contain a number of darker lines. Use the concept of discrete electron energy levels to explain the existence of these darker lines.[4]
(b)The uppermost electron energy bands in a solid are illustrated in Fig. 10.2. [Figure 10.2] Use band theory to explain the dependence on light intensity of the resistance of a light-dependent resistor (LDR).[5]

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Mark scheme

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

[B4] photon gives energy to electron (in an inner shell) or electron (in an inner shell) absorbs a photon; electron moves (from lower) to higher energy level; energy (of photon) is equal to difference in energy levels; electron de-excites giving off photon (of same energy); photons emitted in all directions
[B1] (in light) photons gives energy to electrons in VB / (in light) electrons in VB absorb photons
[B1] electron crosses FB/jumps to CB
[B1] (positive) holes left/created in VB
[B1] low intensity: few electrons in CB/most electrons in VB / high intensity: more photons so more electrons in CB / electron-hole pairs are charge carriers
[B1] more charge carriers results in lower resistance

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