Edexcel A-Level Physics Photon Energy

Practise Edexcel A-Level Physics photon energy with E = hf, wavelength conversions, eV units, LEDs and energy-per-photon reasoning.

Exam points

calculate photon energy from frequency or wavelength, then convert between joules and eV
determine Planck constant from LED wavelength and minimum electron energy data
explain photon number changes by linking shorter wavelength to greater energy per photon

[Maximum number: 6]

A red light-emitting diode (LED) was placed into a circuit, as shown.

Figure for Question 2.3.58 - Photon energy question 1 — Edexcel A-Level Physics AS

The LED emits light of wavelength of 627 nm and may be assumed to be 100% efficient. The battery has negligible internal resistance.

When the switch was closed, the reading on the ammeter was 9.2 mA.

(a)

Calculate the number of photons emitted by the LED in one minute.

Number of photons emitted in one minute =

[ 4 ]

In one minute the LED emits $E=Pt=0.11(60)=6.6\,\mathrm J$ . One photon has energy $hf=hc/\lambda=3.17\times10^{-19}\,\mathrm J$ . Therefore the number emitted is $6.6/(3.17\times10^{-19})=2.1\times10^{19}$ .

(b)

The red LED was replaced with a green LED which emits light with a shorter wavelength than the red LED. The current in the circuit did not change.

Explain how replacing the LED affected the number of photons emitted in one minute.

[ 2 ]

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2.3.58 - Photon energy question 1

Question (a)

Question (b)