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| Electromagnetic Radiation and Energy(Planck's quantum theory) |
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| In 1900, Max Planck presented the results of his famous black body radiation experiments, which showed that light has a dual character, behaving like a particle as well as a wave. He gave the Quantum Theory of Radiation explaining electromagnetic radiation and energy. |
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| Its main features were: |
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 Radiant energy is emitted or absorbed discontinuously in the form of small packets of energy called 'quanta' (and not continuously as thought earlier). Each such quantum is associated with a definite amount of energy. In the case of light, the quantum of energy is called 'photon'. |
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 The amount of energy associated with a quantum of radiation is
proportional to the frequency ( n)
of the radiation. |
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 |
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| where 'h' is proportionality constant, universally referred to as Planck's constant. It has a fixed value of: |
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| h = 6.63 x 10 34 joule.sec or h = 3.99 x 10 -13 kJ sec mol-1. |
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| This relation was found to be valid for all types of electromagnetic radiation. |
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 The total amount of energy emitted or absorbed by a body will be some whole number multiple of the quantum, i.e. E = nh n, where |
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| n = 1, 2, 3, 4, ….. |
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| In other words a body can emit or absorb energy equal to 1h, 2h, 3h etc and not as 1.6h n, 2.6h n, 3.2h n etc. |
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| Thus the energy associated with a quantum of radiation depends inversely on its wavelength (or conversely with its frequency) i.e., higher the wavelength of radiation, lesser the energy associated with its quantum. For example a photon of violet light will have more energy than that of a red light because the former has a lower wavelength. The concept of energy packets of light supports the corpuscular character. |
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| Problem |
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| 5. Calculate the energy of a photon of light having frequency of |
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| 3.0 x 1015 sec-1 (Planck's constant h = 6.63 x 10-34 J s) |
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| Solution |
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| The energy of a photon is given by E = h n |
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| where the frequency of light = 3.0 x 1015 s-1 |
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| Planck's constant, h = 6.63 x 10-34 J s |
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| E = (6.63 x 10-34 J s) x (3.0 x 1015 s-1) |
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| = 6.63 x 3 x 1019 J |
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| = 1.99 x 10-18 J. |
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