The wave nature of radiation suggests that energy is emitted or absorbed continuously as electromagnetic waves. However, wave nature of radiation can not explain the experimentally observed phenomena like photoelectric effect, thermionic emission etc. Max Planck gave his revolutionary theory, called Planck's quantum theory, about the nature of radiation in 1901. According to this theory, the radiant energy is emitted in bundles and not continuously as suggested by wave theory. Each bundle or packet of energy carries a certain amount of energy and travels with the velocity of light. Therefore, a beam of electromagnetic wave can be considered to be streamed of particles, each carrying a certain amount of energy. Planck called these packets of energy as quanta.
Photoelectric Effect
Whenever light or electromagnetic radiation such as X-rays, ultraviolet rays fall on a metal surface, some electrons are emitted from the surface. This phenomenon of emission of electron from a metallic surface when radiation of suitable frequency falls on it is called photoelectric effect. These electrons are called photo electrons.
Metals like Zinc, Cadmium, etc. are more sensitive only to ultraviolet light whereas alkali metals like Sodium, Potassium etc. are sensitive even to visible light.
Quantum Theory of Radiation
The quantum theory of radiation was first proposed by Plank in 1901 to explain the black body radiation. According to this theory, these radiations from the body are emitted in separate packets of energy, each packet is called a quantum of energy. Each quantum carry a defined amount of energy called Photon.
Given by
E=hf
Where f is the frequency of radiation and h is a constant called the Planck constant, whose value is 6.62 * 10 joule sec.
This is the quantum theory of radiation. Therefore, from this relation (i) we know that the photons or quanta with high frequency has a large amount of energy while those of low frequency has less amount of energy.
Experimental Study of Photoelectric Effect
It consists of an evacuated glass or quartz tube with two electrodes A and C. A is the anode and C is the cathode. A constant potential difference is maintained between C and A by a battery. The photoelectric current is measured by a millimeter while the potential difference is measured by a voltmeter V. The tube is evacuated so that the emitting surface is not contaminated by Collins with air molecules and electrons.
When a suitable radiation is incident on the electrode C, electrons are emitted. These electrons get accelerated towards the plate A if it is kept at a positive potential with respect to the cathode. A current thus flows in the outer circuit, which is called photoelectric current.
When a suitable radiation is incident on the electrode C, electrons are emitted. These electrons get accelerated towards the plate A if it is kept at a positive potential with respect to the cathode. A current thus flows in the outer circuit, which is called photoelectric current.
When intensity and frequency of the incident light are kept fixed while potential of the anode is varied, it is found that the photoelectric current increases with the increase in positive potential on the anode till it reaches a value when photoelectric current reaches a saturation value.
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