03-11-2012, 10:47 AM
Photoelectric effect
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Aim of the experiment
To determine Planck’s quantum of action from the photoelectric voltages
measured at different wavelengths.
Apparatus required
Photocell
Interference filter set
Spectral lamp Hg
Power supply for spectral lamps
Mounting plate
Electrometer amplifier
Digital multimeter
Connecting cords
Theory
Inside of the photocell there is a metal coated cathode. The annular anode is opposite
the cathode. When a photon of frequency n strikes the cathode, then an electron can
be ejected from the metal (external photoelectric effect) provided the photon has
sufficient energy.
Some of these ejected electrons reach the (unilluminated) anode so that a potential
difference is set up between anode and cathode, which reaches the limiting value V
after a short (charging) time. The electrons flow against the electric field set up by the
voltage V.
Discussion
i. The view that the light propagates as a series of little packets of energy
(photons) is directly opposed to the wave theory of light. According to the
wave theory, which provides the sole mean of explaining the optical
effects like interference and diffraction, the energy carried out by the light
is distributed continuously through out the wave pattern. According to the
quantum theory, which is strikingly successful in explaining photoelectric
effect, light spreads out from the source as a series of localized
concentration of energy.
ii. In a specific event light exhibits either a wave or a particle nature, never
both simultaneously. The wave theory of light and the quantum theory of
light are complement to each other.