The Electromagnetic spectrum
When a beam of white light enters a prism, it is transformed into a spectrum. We can see the coloured spectrum when it reflects from a surface (see Figure 1).
Figure 1. A white light source entering the triangular prism from the right, and exiting on the left. We see the coloured spectrum when it reflects from the white wall.
The colours we see in the spectrum are a range of vivid hues best known by the acronym ROYGBIV, and are also seen in the rainbow. The Additional Note on RAINBOWS AND THE COLOURS OF THE SPECTRUM describes some of Isaac Newton’s investigations on colour, light and prisms.
Light is a natural phenomenon, generated by objects like the sun, lightbulbs and computer screens, and it travels through space like a wave. Different parts of light’s spectrum have different wavelengths (the distance from one wave’s peak to the next) or energies. The light that we see with our human eyes makes up one small part of the electromagnetic spectrum, and is referred to as visible light by physicists and astronomers, as it is what is visible to the human eye.
Figure 2. The visible spectrum.
There are also wavelengths outside of the visible spectrum which are invisible to the human eye. You are probably familiar with all or most of them: ultraviolet rays, X-rays, gamma rays, microwaves and radio waves. We experience many of these electromagnetic waves in our everyday lives, but we are not able to detect these wavelengths with our eyes. Figure 3 depicts all parts of the electromagnetic spectrum.
Figure 3. Comparison of wavelength, frequency and energy for the electromagnetic spectrum. (Credit: NASA’s Imagine the Universe).
There are a few misconceptions about the visible spectrum which are worth keeping in mind.
Misconception #1
The visible spectrum contains all the colours we can see. This is incorrect. What we see is a range of the most vivid hues that belong to the hue families ranging from violet to red. Pale, dark and muted colours are not part of the visible spectrum, nor are achromatic colours. Also, it is more precise to recognize that the range of colours seen in the visible spectrum is a range of the most vivid hues.
It is important to recognize the difference between hue and colour, and use the appropriate terminology. This difference is discussed in the common misconception: Colour and Hue are the same thing. It is more precise to refer to the hues of the spectrum rather than the colours of the spectrum.
Also note that there are hue families which are not part of the spectrum: magentas, purples, red-purples, etc. These are called non-spectral hues, and are explored in **Exercise 4, and discussed further in the Additional Note YES, MAGENTA IS A ‘REAL’ COLOUR.
Misconception #2
There are only 7 colours in the spectrum: ROYGBIV. This is incorrect. In Examining the Rainbow, we will explore how the visual spectrum depicts a continuous range or gradient of many hues.
Misconception #3
Magenta is not a real colour because it is not part of the spectrum. This is incorrect. Magenta is as ‘real’ as any other hue.
Find out more:
See NASA’s Imagine the Universe and video The Electromagnetic Spectrum.
