A-LevelPhysicsQuantum physics9702/43 · May/June 2020 · Q109 Marks

White light passes through a cloud of cool low-pressure gas, as illustrated in [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 [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 [Figure 10.2]. Use band theory to explain the dependence on light intensity of the resistance of a light-dependent resistor (LDR).[5]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

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

[B4] Any four points, 1 mark each:
- 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 OR (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 OR high intensity: more photons so more electrons in CB OR electron-hole pairs are charge carriers
[B1] more charge carriers results in lower resistance

On Oracle Prep you can write or photograph your own answer and have it marked by AI against this mark scheme, with feedback on what you missed.