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Thermodynamics

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Photo Electric Effect

Hertz, in 1887, discovered that a metallic surface is capable of emitting electrons when light of very short wavelength falls on it. He noted that the air in the spark gap became a better conductor when it was illuminated by ultra-violet light from an arc lamp. Hallwachs, in 1888, found that,

1. When ultra-violet light was incident on a neutral zinc plate, the plate became positively charged:

2. When ultra-violet light was incident on a negatively charged zinc plate, it lost its charged rapidly ; and

3. When ultra-violet light was incident on a positively charged zinc plate, it became more positively charged.

He came to the conclusion that only negatively charged particles can be emitted by the surface under the action of ultra-violet particles was the same as that for cathode rays. Einstein, in 1916, studied the effect of visible light of a range of frequencies on sodium, potassium, cesium, rubidium and lithium and found that the electrons were ejected out of these metals.

The electrons ejected out of the metal under the action of light are known as photo-electrons and this effect is known as photo-electric effect.

Experiment: The photo-electric effect is studied with the help of the following experiment. An evacuated glass tube has a photoelectrically sensitive plate P. It may be a highly polished zinc plate. A hollow cylinder C has a small hole that permits the incident light to fall on the plate P. A quartz window W allows the light to enter the tube. P is connected to the negative and C is connected to the positive terminal of the battery. A sensitive galvanometer used in the circuit as shown in fig.

When light from some source such as a mercury lamp falls on the plate P. Photo-electrons are ejected out of the plate P. These photo-electrons are attracted by the positively charged cylinder C. The galvanometer shows deflection and thus photoelectric current is produced in the circuit. The effect of light for a range of frequencies was studied.

The following results were observed:

1. For a given metallic surface, there is a smallest value of frequency for which the incident light can eject the photo-electrons out of the metal. Light of frequency smaller than this particular value cannot eject electrons, no matter how long it falls on the surface or how high is its frequency.

2. The number of photo-electrons ejected depends upon the intensity of the incident light. Thus, the photo-electric current depends upon the intensity of the incident light.

3. Light frequency greater than the critical frequency or threshold frequency ejects electrons of different velocities. The maximum velocity with which an electron is ejected, depends upon the frequency of the incident light and not on the intensity of the incident light. The threshold frequency is the minimum frequency which can eject an electron out of the metal. Light of frequency less than the threshold frequency cannot eject an electron out of the metal, howsoever intense the light may be.

4. The maximum kinetic energy of the ejected electrons increases linearly with the frequency of the incident light. It does not depend on the intensity of the incident light.

The above experimental facts could not be explained on the basis of wave theory of light. Einstein, in 1905, explained the photoelectric effect based on Planck’s idea of quantum theory of light. According to quantum theory, light consists of quanta or corpuscles of energy hv which move through space with the velocity of light. These particles are known as photons and each photon has an energy = hv where h is the Planck’s constant and v is the frequency of light. When a photon collides with an electron in the metal, it may transfer to the electron. This transfer is an “all or none” process i.e. either the photon gives the whole of its energy hv to the electron or no energy to the electron.