By the end of the 19th century, several types of electromagnetic wave had been discovered:
- radio waves – these were discovered by Heinrich Hertz when he was investigating electrical sparks - infrared and ultraviolet waves–these lie beyond either end of the visible spectrum - X-rays – these were discovered by Wilhelm Röntgen and were produced when a beam of electrons collided with a metal target such as tungsten - γ-rays – these were discovered by Henri Becquerel when he was investigating radioactive substances.
We now regard all of these types of radiation as parts of the same electromagnetic spectrum, and we know that they can be produced in a variety of different ways.

The speed of light

James Clerk Maxwell showed that the speed c of electromagnetic waves in a vacuum (free space) was independent of the frequency of the waves. In other words, all types of electromagnetic wave travel at the same speed in a vacuum. In the SI system of units, c has the value:

$c = 299792458\,m\,{s^{ - 1}}$

The approximate value for the speed of light in a vacuum, which is often used in calculations, is $3.0 \times {10^8}\,m\,{s^{ - 1}}$.
The wavelength λ and the frequency f of the waves are related by the equation:

$c = f\lambda $

This is the same as the wave equation: the wave speed $v = c$. When light travels from a vacuum into a material medium such as glass, its speed decreases but its frequency remains the same, and so we conclude that its wavelength must decrease. We often characterise different forms of electromagnetic by their different wavelengths. But it is better to characterise them by their different frequencies. That’s because their wavelengths depend on the medium through which they are travelling.