Maxwell’s suggestion about the wave nature of electromagnetic radiation was helpful in explaining the phenomena such as interference, diffraction etc. However, he failed to explain various other observations such as the nature of emission of radiation from hot bodies, photoelectric effect i.e. ejection of electrons from a metal surface when electromagnetic radiation strikes it, the dependence of heat capacity of solids upon temperature, line spectra of atoms (especially hydrogen). Let’s learn about Planck’s quantum theory.
Black Body Radiation :
Solids, when heated, emit radiation varying over a wide range of wavelengths. For example: when we heat solid color changes which continue to change with a further increase in temperature. This change in color happens from a lower frequency region to a higher frequency region as the temperature increases. For example, in many cases, it changes from red to blue. An ideal body which can emit and absorb radiation of all frequencies is called a black body. The radiation emitted by such bodies is called black body radiation.
Thus, we can say that variation of frequency for a black body radiation depends on the temperature. At a given temperature, the intensity of radiation is found to increase with an increase in the wavelength of radiation which increases to a maximum value and then decreases with an increase in the wavelength. This phenomenon couldn’t be explained with the help of Maxwell’s suggestions. Hence Planck proposed the Planck’s quantum theory to explain this phenomenon.
Planck’s quantum theory :
According to Planck’s quantum theory,
- Different atoms and molecules can emit or absorb energy in discreet quantities only. The smallest amount of energy that can be emitted or absorbed in the form of electromagnetic radiation is known as quantum.
- The energy of the radiation absorbed or emitted is directly proportional to the frequency of the radiation. The energy of radiation is expressed in terms of frequency as,
E = h ν
E = energy of the radiation
h = Planck’s constant (6.626×10–34 J.s)
ν= Frequency of radiation
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