A-LevelPhysicsQuantum physics9702/43 · May/June 2018 · Q118 Marks

Explain what is meant by a photon.

(a)(i)Explain what is meant by a photon.[2]
(a)(ii)By reference to intensity of light, state one piece of evidence provided by the photoelectric effect for a particulate nature of light.[1]
(b)Some electron energy levels in a solid are illustrated in Fig. 11.1. [Figure 11.1] A semiconductor material has a very high resistance in darkness. Light incident on the semiconductor material causes its resistance to decrease. Explain the resistance of the semiconductor material in different light conditions.[5]

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Mark scheme

Marking points from the official mark scheme (May/June 2018). The mark or mark type is shown in brackets.

[M1] packet/quantum/discrete amount of energy
[A1] of electromagnetic radiation
[B1] (maximum) energy of emitted electrons is independent of intensity or no emission of electrons below the threshold frequency regardless of intensity or no emission of electrons when photon energy is less than work function (energy) regardless of intensity
[B1] in darkness: conduction band empty so high resistance
[B1] in daylight: electrons in valence band absorb photons
[B1] in daylight: electrons 'jump' to conduction band
[B1] this leaves holes in valence band
[B1] more charge carriers in daylight so resistance decreases

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