Electromagnetic radiation is radiation, also known as radiant energy, consisting of vibrating electric and magnetic fields, which propagates at the speed of light (300,000 kilometers per second in a vacuum). Electromagnetic radiation may also be regarded as either a series of electromagnetic waves or as a stream of particles known as photons. Radiant energy is emitted from objects when they are heated (see blackbody) or otherwise energetically excited, as in the case of luminescence.

 

An electromagnetic wave consists of electric and magnetic fields vibrating at right-angles to each other.

Classically, radiant energy is considered to be a form of wave motion. In the mid-nineteenth 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 twentieth century Max Planck proposed that certain properties of radiation were best explained by regarding it as transporting energy in discrete amounts called quanta. The name 'photon' was adopted to describe the quantum of the electromagnetic field. The energy of each photon is proportional to the frequency of the associated radiation. In general, the higher the energies involved, the better the properties of the radiation are described in terms of particles rather than waves.

 

Electromagnetic spectrum

The different kinds of electromagnetic radiation are classified according to the wavelengths (or frequencies) of the waves (or, equivalently, the energies of the photons) involved. The range of wavelengths (or energies) is known as the electromagnetic spectrum.

 

The electromagnetic spectrum spans the entire range of electromagnetic radiation from the longest wavelengths (lowest frequencies) of 1,000 m or more to the shortest wavelengths (highest frequencies) of about 10^-15 m. It includes:

 

radio waves
microwave radiation
infrared
visible light
ultraviolet
X-rays
gamma rays