Demerits of Electromagnetic Theory

It could not explain the following phenomena

● Black -body radiation● Photoelectric effect● variation of heat capacity of solids as a function

of temperature ● line spectra of atoms with special reference to

hydrogen.

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Black body radiation

● The ideal body, which emits and absorbs radiations of all frequencies, is called a black body and the radiation emitted by such a body is called black body radiation.

● The exact frequency distribution of the emitted radiation from a black body depends only on its temperature.

● At a given temperature, intensity of radiation emitted increases with decrease of wavelength, reaches a maximum value at a given wavelength and then starts decreasing with further decrease of wavelength

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Wavelength – Intensity Relationship

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Photoelectric Effect

● In 1887, H. Hertz performed a very interesting experiment in which electrons were ejected when certain metals like potassium, rubidium, caesium etc were exposed to a beam of light .

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ObservationsThe results observed in this experiment were:

● The electrons are ejected from the metal surface as soon as the beam of light strikes the surface.

● The number of electrons ejected is proportional to the intensity or brightness of light.

● For each metal, there is a characteristic minimum frequency, ν0. (threshold frequency) below which photoelectric effect is not observed.

● At a frequency ν >v 0, the ejected electrons come out with certain kinetic energy. The kinetic energies of these electrons increase with the increase of frequency of the light used.

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Planck's Theory of Radiations

● In order to explain these facts Planck (1901) gave a theory called Planck's quantum theory of radiation.

● Later this theory was applied to light by Einstein and proved that light like other electromagnetic radiations exhibited dual nature, i.e., wave and particle nature

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Planck's Theory of Radiations

The various postulates of quantum theory of radiations are as follows:

● A radiation has energy. As light and heat are radiations, they are also associated with energy.

● Radiant energy is not emitted or absorbed continuously but discontinuously in the form of small packets called photons. Photon is not a material body but is considered to be a massless packet of energy.

● The energy E of a photon is related to the frequency of radiation, n; the two

being related as E = hν Where h is planck's constant.

● Whenever a body emits or absorbs energy, it does so in whole number multiples of photons, i.e., nhν where n = 1, 2, 3 and never

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Einstein was able to explain the photoelectric effect using Planck's quantum theory of electromagnetic radiation ● Shining a beam of light on to a metal surface can, therefore, be

viewed as shooting a beam of particles, the photons. ● When a photon of sufficient energy strikes an electron in the

atom of the metal, it transfers its energy instantaneously to the electron during the collision and the electron is ejected without any time lag or delay.

Explanation

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● Greater the energy possessed by the photon, greater will be transfer of energy to the electron and greater the kinetic energy of the ejected electron.

● In other words, kinetic energy of the ejected electron is proportional to the frequency of the electromagnetic radiation.

● Since the striking photon has energy equal to h and the minimum energy required υ

to eject the electron is hυo (work function)

● then the difference in energy (h - hυ υo) is transferred as the kinetic energy of the photo electron. Following the conservation of energy principle, the kinetic energy of the ejected electron is given by the equation.

Explanation

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Exercises

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Exercises

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Exercises

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Exercises