Classically, radiant energy is considered to be a form of wave motion. In the mid-19th century, James Clerk Maxwell showed that an oscillating electric charge would be surrounded by varying electric and magnetic fields. Energy would be lost from the oscillating charge in the form of transverse waves in these fields, the waves in the electric field being at right-angles to those in the magnetic field and to the direction in which the waves are traveling. Moreover, the velocity of the waves would depend only on the properties of the medium through which they passed; for propagation in a vacuum its value is a fundamental physical constant of physics – the electromagnetic constant, c.
At the beginning of the 20th century Max Planck proposed that certain properties of radiation were best explained by regarding it as transporting energy in discrete amounts called quanta. Albert Einstein later proposed the name photon for the electromagnetic quantum. The energy of each photon is proportional to the frequency of the associated radiation.
The different kinds of electromagnetic radiation are classified according to the energy of the photons involved, the range of energies being known as the electromagnetic spectrum. In general, the higher the energies involved, the better the properties of the radiation are described in terms of particles (photons) rather than waves.
Radiant energy is emitted from objects when they are heated (see blackbody) or otherwise energetically excited (see luminescence; spectroscopy).
Related categories ELECTRICITY AND MAGNETISM
WAVES AND WAVE PHENOMENA
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