CAIE A-Level Physics 22.2 Photoelectric Effect

Practise analysing photoelectric emission, threshold quantities, photon energy, work function, Einstein’s equation and graph effects of frequency and intensity.

Exam points

explain photoelectric emission and determine threshold conditions from frequency or wavelength
use photon energy and work function to explain the energy of emitted electrons
calculate or rearrange the Einstein photoelectric equation for maximum kinetic energy
interpret photoelectric graphs and distinguish frequency effects on electron energy from intensity effects on current

Covered syllabus points

22.2.1—Photoelectrons may be emitted from a metal surface when it is illuminated by 22.2.2—The terms threshold frequency and threshold wavelength 22.2.3—Photoelectric emission in terms of photon energy and work function energy 22.2.4—Hf = Φ + 1/2 mvmax 2 22.2.5—Why the maximum kinetic energy of photoelectrons is independent of

[Maximum number: 7]

(a)

Explain what is meant by the photoelectric effect.

[ 2 ]

(b)

Light from the lamp in (b) is incident, separately, on the surfaces of caesium and tungsten metal.

Data for the work function energies of caesium and tungsten metal are given in Fig. 10.1.

Fig. 10.1

Calculate the threshold wavelength for photoelectric emission from

[ 3 ]

(i)

caesium, nm

[ 2 ]

$\Phi=h c / \lambda_{0}$

\begin{aligned} \lambda_{0} & =\left(6.63 \times 10^{-34} \times 3.00 \times 10^{8}\right) /\left(1.4 \times 1.60 \times 10^{-19}\right) \\ & =890 \mathrm{~nm} \end{aligned}

(ii)

tungsten.

threshold wavelength =nm [1]

[ 1 ]

$\lambda_{0}=\left(6.63 \times 10^{-34} \times 3.00 \times 10^{8}\right) /\left(4.5 \times 1.60 \times 10^{-19}\right)$

=280 \mathrm{~nm}

(c)

Use your answers in (c) to state and explain whether the radiation from the mercury lamp of wavelength 436 nm will give rise to photoelectric emission from each of the metals.
caesium:
tungsten:

[ 2 ]

caesium:
wavelength of photon less than threshold wavelength (or v.v.)
or
$\lambda_{0}=890 \mathrm{~nm}>436 \mathrm{~nm}$
so yes
tungsten:
wavelength of photon greater than threshold wavelength (or v.v.)
or
$\lambda_{0}=280 \mathrm{~nm}<436 \mathrm{~nm}$
so no

[Maximum number: 5]

(a)

By reference to the photoelectric effect, explain what is meant by work function energy.

[ 2 ]

(b)

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

[ 2 ]

(i)

Determine the work function energy in J of the metal used in the experiment.
work function energy = J

[ 2 ]

Correct read off from graph of f as $5.45 \times 10^{14} \mathrm{~Hz}$ when $E_{\text {MAX }}=0$ $5.45 \times 10^{14} \times 6.63 \times 10^{-34}$

C1

$=3.6 \times 10^{-19} \mathrm{~J}$

A1

(c)

The intensity of the electromagnetic radiation for one particular frequency in (b) is increased.

State and explain the change, if any, in:

[ 1 ]

(i)

the rate of emission of photoelectrons.

[ 1 ]

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Covered syllabus points

22.2 Photoelectric effect question 1

Question (a)

Question (b)

Question (i)

Question (ii)

Question (c)

22.2 Photoelectric effect question 2

Question (a)

Question (b)

Question (i)

Question (c)

Question (i)