‘Wholesale’ photoelectric effect
Practical Activity for 14-16
This demonstration shows students that light has a particle property: it packages its energy in small packets or quanta.
Apparatus and Materials
- Zinc-plate attachment
- EHT power supply with large series safety resistor
- Wire mesh
- Fine emery cloth, 1 piece
- Ultraviolet lamp (a
blacklightUV lamp will not do - use one with a clear quartz envelope)
- Connecting leads
- Flexicam or webcam linked to a projector (OPTIONAL)
- Lamp, 12 V, 24 W and power supply (OPTIONAL) [Not a modern tungsten-halogen compact lamp which emits some UV]
Health & Safety and Technical Notes
The ultraviolet lamp should have a shield with a fairly small aperture (1 or 2 cm in diameter) so that neither you nor your students can see into the light source. Do not look into the ultraviolet lamp, or expose skin to the UV radiation.
To help students see the gold leaf, you could use a flexicam or webcam connected through a computer to a projector, or you could use the lamp to cast a shadow of the gold leaf. (Remove the back plate of the electroscope and place the lamp below the leaf to cast a higher shadow.)
It may help if pupils have seen photocells at work in electric or electronic circuits, where light releases a horde of electrons from a sensitive surface in a vacuum and the horde acts as a current to do jobs for us. That might be called the ‘wholesale photoelectric effect’.
The EHT supply has a floating earth. You can use this to earth the case of the electroscope. Then, to charge the plate negatively, connect the EHT supply’s positive terminal to its earth, and use its negative terminal to charge the plate. To charge the plate positively, connect the EHT supply’s
negative terminal to its earth, and use its positive terminal to charge the plate.
- Set up the gold leaf electroscope so that the pupils have a clear view of the leaf.
- Thoroughly clean the zinc plate with the emery cloth and attach it to the electroscope.
- Support the wire mesh a few centimetres away from the zinc plate. This mesh is connected to the case of the electroscope, which is earthed.
- Use the EHT supply to charge the plate on the electroscope negatively.
- Switch on the ultraviolet lamp so that it illuminates the plate. Observe the effect on the gold leaf.
- Repeat the experiment with the plate and electroscope charged positively.
- Finally, repeat the experiment with the plate negatively charged. When the charge is clearly seen to be leaking away, put a sheet of glass between the light source and the charged plate.
- The negatively charged zinc plate illuminated by ultraviolet radiation loses charge, suggesting that the radiation is ejecting electrons from its surface.
- It only loses charge when it is negatively charged. This suggests that it is negative charges that are ejected by the ultraviolet radiation. They are helped by an electric field which is negative at the plate, but they are held back by an electric field which is positive at the plate. You can tell students that more complex experiments can show that the particles ejected are electrons.
- The piece of glass absorbs ultraviolet radiation. When it reduces the ultraviolet radiation, the discharge slows. This shows that it is the ultraviolet radiation that ejects the electrons from the zinc.
- Normal, visible, light does not release electrons – however long it is left and whatever its intensity. You can show this using a lamp or by referring to the ambient light.
- When the ultraviolet lamp is first turned on there is no delay in the production of electrons. If the discharge were relying on a continuous stream of light, there would need to be a delay to build up enough energy in the metal to eject each electron. The fact that there is no delay is evidence that the energy arrives in quanta.
- More complex experiments can show that the radiation (whether visible or ultraviolet) arrives in quanta. You can say that these are called photons.
- Refer students to the clicks they hear when gamma radiation is detected by a Geiger-Müller tube. The radiation is part of the electromagnetic spectrum but is clearly arriving at the Geiger-Müller tube in quanta.
- The experiment shows that it is the wavelength of the radiation that is important rather than the intensity. You can tell students that more complex experiments can show that the energy of a quantum is given by E = hf.
This experiment was safety-tested in April 2006