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Light exhibits dual nature (wave and matter both), it means certain natural phenomenon can be explained assuming light as a wave but to explain certain other we need to assume the light is made up of particles. The energy of light is contained by energy packets, the energy content of a packet is fixed and depends on frequency of the wave. The energy packet is called photon.
Thermal radiation
The certain minimum energy is required for the emission of electron from the metal surface can be supplied to the free electrons by the physical process called thermionic emission (thermal radiation). The thermal radiation- “by suitably heating, sufficient thermal energy can be imparted to the free electrons to enable them to come out of the metal.”
The photoelectric effect
Hallwachs and Lenard obresved that when ultraviolet light fell on the emitter plate, no electrons were emitted at all when the frequency of the incident light was smaller than a certain minimum value, called the threshold frequency. This minimum frequency depends on the nature of the material of the emitter plate. Theoretically the photoelectric effect is explained by Elbert Einstein for which he was awarded by Nobel Prize. Photo electric effect supports the particle nature of light. It states “when suitable energy (frequency) of photo strikes on the metal surface the electrons from the metal will come out, phenomenon is explained as following on behalf of following figure:
1) Certain part of the incident energy ( E = hv ) spend against the work function ( W = hv0 ) of the metal.
2) Rest part of incident energy constitute the kinetic energy ( K.E. = hv – hv0 ) of the emitted electrons.
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Quantum Physics