The electroscope is given a negative charge and the gold leaf diverges as indicated. When ultra violet light is then shone on the zinc plate, the gold leaf is seen to collapse, showing that the charge has been lost. This is due to electrons being emitted by the zinc. The phenomenon is called the photoelectric effect, and the electrons are called photoelectrons. Zinc is said to have a ‘photoemissive surface’ when exposed to ultra violet light. Some materials are photoemissive in visible light.
When electromagnetic (‘em’) radiation of suitable wavelength is shone on the cathode it emits electrons. If the anode is made positive, it collects the electrons and a current flows.
Using a set-up like the above, the following three laws have been discovered:
1. If the frequency of the incident radiation is reduced, there is a minimum frequency, called the threshold frequency, below which no current is detected, therefore no electrons are being emitted, no matter how intense the radiation. The threshold frequency depends on the material of the cathode - for zinc the frequency is in the ultraviolet.
2. For frequencies above the threshold frequency, the current, and therefore the number of electrons emitted per second, is proportional to the intensity of the incident radiation (E.g. - doubling the intensity, doubles the current).
3. For frequencies above the threshold frequency, if the anode is made negative it repels the electrons and, at a certain p.d., called the stopping potential,Vs, the current becomes zero. With the current just stopped, it is found that increasing the intensity of the radiation does not restart it. Also, the value of VS is found to increase with the frequency of the radiation. This implies that electrons are emitted with a range of kinetic energies, up to a certain maximum value, a value which depends only upon the frequency of the incident radiation and not upon its intensity.
To explain certain phenomena we have to assume that electromagnetic radiation…