Topic Menu

Cyber-chess a chess instruction site
Questions on the Photoelectric Effect

Q2.

The graph shows how the maximum kinetic energy of electrons emitted from the cathode of a photoelectric cell varies with the frequency of the incident radiation.

(a) Calculate the maximum wavelength of electromagnetic radiation that can release
photoelectrons from the cathode surface.

The maximum wavelength corresponds to the minimum frequency of the photon. This can be read directly off the graph.

You can then use of c = fλ to find the value of the wavelength.

f = 4.0 x 1014 Hz.

c = fλ

λ = c/f = 3.0 x 108/4.0 x 1014

λ = 7.5 x 10-7 m

(3 marks)

(b) Another photoelectric cell uses a different metal for the photocathode. This
metal requires twice the minimum energy for electron release compared to the
metal in the first cell.

(i) If drawn on the same axes, how would the graph line obtained for this second
cell compare with the one for the first cell?

The line would be parallel to the original line, but the intercept on the frequency axis would be at 8.0 × 1014 Hz

(ii) Explain your answer with reference to the Einstein photoelectric equation.

You need to compare EK = hf –Φ with the straight line equation y = mx + c, it should be clear that h is the gradient (m). h is a constant, so the gradient is unchanged. When EK = 0, f is the threshold frequency, where hf0 = Φ.

If the work function Φ is doubled, so will be the threshold frequency.

The gradient of the line is the Planck constant, h, so it is unchanged and the intersection with the f-axis is doubled because hf0 = Φ when the photoelectrons have zero kinetic energy - in this case the energy would be twice that of the original if f doubles.

(3 marks total)

(Total 6 marks)