Last revised by Daniel J Bell on 26 Nov 2021

Electromagnetic radiation refers to the waves or quanta of the electromagnetic field as they propagate through space. The speed of electromagnetic waves is invariant in a vacuum, being ~3x108 m/s and represented by the symbol, c, otherwise known as the speed of light. The types of electromagnetic radiation vary from one another in their wavelength and hence also in energy and frequency. The approximate relationship between the frequency, wavelength and energy is given by

• E = 4.14 × 10-15 f
• λ = 3.00 × 108 / f
• E = 1.24 × 10-6 λ

where E is the energy in electron-volts, λ is the wavelength in meters and f is the frequency in hertz. The choice of which quantity to use depends on where in the electromagnetic radiation spectrum the specific type of radiation is found.

#### Types

Electromagnetic radiation is often classified into seven different types, primarily defined by their wavelengths 2:

• radio waves: 100 Hz to 1 MHz / 3000 m to 3 m
• microwaves: 1 MHz to 1 GHz / 3 m to 0.3 mm
• infrared: 1 mm to 700 nm / 0.001 eV to 1.77 eV
• visible light: 700 nm to 400 nm / 1.77 eV to 3.10 eV
• ultraviolet: 400 nm to 3 nm / 3.10 eV to 413 eV
• x-rays: 41 eV to 413 keV
• gamma rays: 10 keV to >100 GeV

It should be noted that these boundaries are not absolute and that there is some overlap. In particular, x-rays and gamma rays are differentiated by their process of creation, not how energetic they are.

A subset of the more energetic rays, primarily x-rays and γ-rays have the potential to ionize atoms, forming part of ionizing radiation.

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