Lesson 12 - Colour and Light
A large part of the beauty and value of gemstones and precious metals revolves around the interaction between light
and the object. This includes not only the hue and saturation of colour, but also how light is transmitted, reflected,
refracted, fluoresced, and dispersed.
Light is electromagnetic radiation or energy, and can be described as behaving like both waves and particles (photon).
Like all waves, light can be described by its wavelength, the distance from peak to peak or trough to trough, and its
frequency, the number of wave crests (or troughs) that pass through one point in one second. Light propagates in the
direction of its wave front. All electromagnetic radiation (from radio waves to x rays) travels at a constant speed. So,
when the frequency of light is decreased, its wavelength must increase
this is an inverse relationship. Light energy
increases with increasing frequency (or decreasing wavelength). Refer to the figure and table below for more
information on the parts of a wave.
Light also behaves like a particle when it travels as photon particles. More intense light would be composed of a greater
number of photons with a higher frequency of incidence. For gemstones, interaction with light is best described using
the wave-like approach. For those students interested in the wave-particle duality of light, a good online starting point is
What is Colour?
Colour is what our brain interprets from the incidence of light (electromagnetic radiation within the visible spectrum) on
our eye. In other words, the colour of an object is our eye's interpretation of light in the visible range that has interacted
with the object we are looking at.
The electromagnetic spectrum is continuous and represents radiation energy ranging from high intensity gamma rays
(short wavelength, high frequency) to low intensity radio waves (long wavelength, low frequency). In the middle of this
is the visible region which ranges from about 350 to 750 nanometers (nm). This range comprises the visible rainbow
with which we are all familiar with: violet at the short end (~400 nm) and red at the long end (~700 nm).
Remember the acronym ROY G. BIV? We use it to remember the sequence of colours of the visible spectrum: Red -
Orange - Yellow - Green - Blue - Indigo - Violet. Just outside of the visible region on the shorter wavelength end is the UV
(ultraviolet) range and NIR range (near infrared) on the longer wavelength end. White does not appear as a colour in the
spectrum because white light is a mixture of light with wavelengths across the visible range.
Different sources of energy will emit electromagnetic radiation at different intensities across the electromagnetic
spectrum. A simple example of this is a light emitting diode (LED) made to emit only one colour (monochromatic). In the