1. Light and Radiation

The radiation spectrum    (1/3)

Wavelength and frequency are parameters which are commonly used to characterise the spectral range of light and radiation. The range of radiation wavelengths and frequencies covers many orders of magnitude, from the radio waves having wavelengths of kilometres to metres up to the x-rays and γ-rays (or: gamma rays) which have wavelengths as small as an atom or even less than an atom.

The spectrum of light and radiation over the frequency and wavelength in logarithmic scales.

Almost hidden in the radiation spectrum in the left column, visible light takes up only a very small portion at around one micrometre (μm) wavelength, bounded by radiation which is invisible to the eye. A closer look at this visible part is shown below.

       

The spectrum of visible light at wavelengths from 400 to 750 nm (or: 0.4 to 0.75 μm) and the play of colours which is seen by the eye in this visible wavelength range.

In these graphs, wavelengths are given in different units:

• centimetres (cm) and metres (m) are used with microwaves and radio waves
• micrometres (μm) are used with infrared radiation, and it is 1 μm=10^-6 m
• nanometres (nm) are used to characterise visible wavelengths, with 1 nm=10^-9 m, and it is 1 μm=1000 nm
• the symbol Å stands for the Ångström unit, with 1 Å=10^-10 m Ångströms are used with x- and γ rays, and it is 1 nm=10 Å.

Frequencies are given in

• terahertz: 1 THz=10¹² Hz
• gigahertz: 1 GHz=10⁹ Hz
• megahertz: 1 MHz=10⁶ Hz
• kilohertz: 1kHz=10³ Hz.

Radio waves have frequencies of around 100 kHz to 100 MHz, microwaves of 1 to 100 GHz, typically. Visible light has frequencies around 500 THz.