CAIE A-Level Physics A2 22.1 Energy and Momentum of a Photon Questions

Practise analysing photon quanta, calculating energy, wavelength, momentum and radiation force, and using power to find photon emission rate.

Exam points

distinguish particulate nature and photon quanta in electromagnetic radiation
calculate photon energy with E = hf or E = hc/? and convert between joules and electronvolts
use photon energy and power to determine photon emission rate
calculate photon momentum and radiation force using p = E/c and momentum change

Covered syllabus points

22.1.1—Electromagnetic radiation has a particulate nature 22.1.2—A photon is a quantum of electromagnetic energy 22.1.3—E = hf 22.1.4—The electronvolt (eV) as a unit of energy 22.1.5—A photon has momentum and that the momentum is given by p = E / c

[Maximum number: 7]

(a)

State what is meant by a photon.

[ 2 ]

(b)

For an accelerating voltage of 32 kV in Fig. 8.1, determine:

[ 5 ]

(i)

the maximum momentum of an X-ray photon produced at the target

maximum photon momentum =

[ 2 ]

momentum =E / c
momentum $=\left(0.032 \times 1.60 \times 10^{-13}\right) /\left(3.00 \times 10^{8}\right)$ $=1.7 \times 10^{-23} \mathrm{~N} \mathrm{~s}$

(ii)

the minimum wavelength of X-rays produced at the target.

minimum wavelength =m [3]

[ 3 ]

E=h f and $\lambda=c / f$ $\begin{aligned} \lambda =h c / E =\left(6.63 \times 10^{-34} \times 3.00 \times 10^{8}\right) /\left(0.032 \times 1.60 \times 10^{-13}\right) \end{aligned}$ $\lambda=3.9 \times 10^{-11} \mathrm{~m}$

[Maximum number: 1]

In an experiment, electromagnetic radiation of frequency f is incident on a metal surface.

The results in Fig. 10.1 show the variation with frequency f of the maximum kinetic energy $E_{\text {MAX }}$ of electrons emitted from the surface.

Fig. 10.1

The work function energy in eV for some metals is given in Table 10.1.

Table 10.1

Determine the metal used in the experiment. Show your working.

$3.6 \times 10^{-19} / 1.6 \times 10^{-19}=2.3 \mathrm{eV}$ so potassium

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Covered syllabus points

Question 1

Question (a)

Question (b)

Question (i)

Question (ii)

Question 2