(a)(i) Explain photoelectric emission.
(ii) State four applications of photoelectric emission.
(b) Draw and label a diagram showing the structure of a simple type of a photocell and explain its mode of operation.
(c) In a photocell, no electrons are emitted until the threshold frequency of light is reached.
(i) Explain what happens to the energy of the light before emission of electrons begin.
(ii) State one factor that may affect the number of emitted electrons.
Explanation
(a)(i) Photoelectric emission is the ejection of electrons from the surface of a metal when light of appropriate wavelength and frequency is incident on the metal.
(ii) Applications of photoelectric emission i.e. photo cells:
- Burglar alarms
- Photo meters
- Automatic doors
- Sound production from film tracks
- T.V. camera etc
(b)
When light of a set wavelength and frequency falls on the cathode, electrons are emitted. The electrons flow toward the anode. The flow of these electrons constitute current.
(c) (i) For energy of light with frequency less than threshold frequency, complete absorption takes place. The energy absorbed is used to raise the energy of electrons or overcome the potential barrier in the photoelectric material.
(ii) The number of emitted electrons are affected by:
- intensity
- Number of photons.