It
is the emission of electrons from the surface of certain substances, mainly
metals, when they are illuminated by electromagnetic radiations like X-rays,
ultraviolet and even visible light. The electrons emitted are called photo electrons.
Experiment to
Demonstrate Photoelectric Effect:
In
figure (a) A and C are two zinc plates, connected to positive and negative of a
battery E respectively, through a micro-ammeter M and a key K. The plates A and
C are enclosed in an evacuated quartz bulb. The quartz will not absorb
ultraviolet rays. It is evacuated, so that, the metal surface remains pure.
The micro ammeter M registers a current, whenever ultraviolet rays fall on the zinc
plate C. These electrons travel towards the plate A and thus a current flows
through the circuit. If ultraviolet rays are stopped, there is no deflection in
the micro ammeter. This means, electrons are ejected from the plate C by the
radiations falling on it.
Laws of Photo Electric
Emission:
⇒ For a given photosensitive
material, there is a minimum frequency below which there is no photoelectric
emission. This frequency is independent of the intensity of light and is called
the threshold frequency.
⇒ Photoelectric
emission is an instantaneous phenomenon. There is no time lag between the
incidence of radiation and the emission of photo electrons.
⇒ The number of
photo electrons and the photoelectric current is directly proportional to the
intensity of incident radiation (provided the frequency
is greater than the threshold frequency).
⇒ The kinetic
energy of photo electrons increases with the frequency of the incident radiation
and is independent of the intensity of radiation.
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