The substance of light
LIGHT is a form of 'radiant energy', made of the same stuff as radio waves, microwaves, x-rays etc. Each of these wave types has a different frequency or wavelength. This is illustrated in fig.1, in which we see that light occupies only a very small part of the total range of radiation. We also see that the spectrum of colours (red, orange, yellow, green, blue, indigo and violet) is spread over a comparatively narrow band of wavelengths ─ red being of the longest and violet of the shortest wavelength. We also see from fig.1 that shorter wavelengths (higher frequencies) have more energy than longer wavelengths (lower frequencies) ─ e.g. gamma rays are more 'energetic' than radio waves. Similarly, violet light carries more energy than red light; it is ultraviolet, not infra-red that gives you sunburn!
Fig.1 The visible spectrum
In air, light travels about 300,000 km (186,000 miles) per second, equivalent to encircling the Earth about 7 times in one second. In water light travels at about 3/4 of this speed. It is this difference that gives rise to the refraction that takes place as light passes through an air-water surface, (see the article: Refraction of light in water in this series).
Light is sometimes considered to travel in small packages of radiant energy called photons. This is illustrated in fig.2 in which you will see low energy (long wave) red photons, higher energy (shorter wavelength) green photons, and highest energy (shortest wavelength) blue photons.
Fig.2 Light traveling as 'photons'
Date of last revision: 25 September 2017