Radiant energy is the energy of electromagnetic waves. It is proportional to the square of the amplitude of the electrical or magnetic field strength. Electromagnetic waves of high frequency and therefore high energy have the character of particles. The energy of these particles is proportional to the frequency and inversely proportional to their wavelength. The proportionality factor is Planck’s quantum of action h. Max Planck discovered in his investigation of radiation from black bodies that radiation energy must be composed of discrete quanta. He formulated a law of radiation which could not, however, be explained until Einstein postulated light quanta.

Numerical example of Planck’s radiation formula:
The sun has a surface temperature of 5,800 K; the associated radiant power according to Planck’s radiation formula is 3.85 x 1023 kW. Only a very small proportion of this strikes the earth (with vertical radiation incidence 1.37 kW/m²).

Information and ideas:
Radiant energy can be converted into other energy forms. In the case of an X-ray radiograph the radiant energy is converted into chemical energy (blackening of the photo film), light is converted into electrical energy in a solar cell, and similarly radio waves in an antenna. The energy of microwaves can be used for heating food.

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