PARTICAL PROPERTIES OF WAVES

Photoelectric effect
-Photoelectric effect is the emission of electrons from metal surface when they absorb energy from an electromagnetic wave.

-These electrons emitted from metal surface are known as photo electrons.

(a) Observed features:
-For a given metal or photosenstive surface there is a minimum frequency ν0 of incident radiation , below which there is no emission of photo electrons and this frequency is called threshold frequency of that metal surface.
-Electronic emission increases with the intensity of radiation falling on the metal surface , since more energy is available to the release of electrons but maximum kinetic energy KEmax (=eV where V is the cut off voltage) is independent of the intensity of light.
-There is no time delay between the arrival of light on the metal surface and the emission of electrons.
-The maximum kinetic energy of photoelectrons and frequency of incident light are related linearly as
KEmax=eV ∝ (ν-ν0)
(b) Theoretical explaination:
-Classical theory which assumes light as an EM wave fails to explain photoelectric effect.
-Einstein first gave correct explaination of photoelectric effect using Plank's idea of energy quantization.
-Einstein in his theory considered that radiation of frequency ν consists of a stream of discrete quanta (photons) each of energy hν , where h is the plank's constant. The photons moves through space with the speed of light.
-When a quanta of energy hν is incident on a metal surface , the entire energy of the photon is absorbed by a single electron without any time lag.
-The minimum amount of work or energy necessary to take a free electron just out of metal against attractive forces of surrounding positive ions is called work function of the metal and is denoted by φ0.
-Appliying principle of conservation of energy to the absorption of photon by an electron in the metal surface we get,
hν=KEmax0
writing φ0=hν0 , we get
KEmax=h(ν-ν0)
-thus maximum kinetic energy is same as observed KEmax with the proportionality constant equal to Plank's constant.

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