Thursday, 9 December 2010

Einstein suggestes quantum nature of Light - Photoelectric Effect

Photoelectric Effect - Another Problem for Classical Physics
Photoelectric effect is the emission of electrons from a metal surface by gaining energy from light.
First observed by Heinrich Hertz (1857 – 94) in 1887
Studied in detail by Philipp Lenard(1862 – 1947) in 1902
Lenard was awarded Nobel Prize in 1905

According to classical physics, the metal surface simply soaked up light energy and sooner or later, depending on the intensity of the light, it would accumulate enough energy to cause emission of electrons.
Experiments did not verify this and cast doubt on the validity of the wave theory of light – a cornerstone of Maxwell’s theory of electromagnetic waves

Experimental results were:
1. If the light is below a certain frequency f0 , then no electrons would be produced however long we shine it on the metal surface.
2. For light frequency above f0 electrons would be produced immediately, with no time delay even for extremely weak light intensities

In 1900, to explain a problem with black-body radiation spectrum Planck had made an ad hoc assumption that emission and absorption of energy can occur only in discrete amounts.
Einstein made the bold assumption that light consists of a stream of particles; the energy of individual particles or quanta is determined by the frequency of light. Einstein’s light quanta are now called photons
Energy of a photon is E = h x f where h is the Planck’s constant.

According to Einstein, in photoelectric emission, a light photon penetrates the metal and knocks an electron loose. A minimum energy is required to knock off an electron from the metal. Hence photons below a certain frequency f0 do not have sufficient energy (= hf0) to release electrons. Increasing the intensity of the light increases the number of photons and hence the number of electrons emitted would increase.

Einstein’s theory raises a fundamental question:
Is light a wave, or a stream of photons?
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