The Visible Spectrum

The visible spectrum is the small slice of the electromagnetic spectrum that human eyes can see. It runs from red (the longest wavelength we can detect) to violet (the shortest), passing through orange, yellow, green, blue and indigo on the way. White light, like sunlight, is actually a mix of all these colours. A prism (or a raindrop, in the case of a rainbow) bends each colour by a slightly different amount, separating them so we can see them as a rainbow band. Beyond the visible range are many other types of light that we cannot see directly, but which surround us all the time.

  • Range400-700 nmA tiny slice of EM spectrum
  • MnemonicROYGBIVRed, Orange, Yellow, Green, Blue, Indigo, Violet
  • Longest visibleRed (around 700 nm)Just before infrared
  • Shortest visibleViolet (around 400 nm)Just before ultraviolet
  • White lightAll colours mixedSunlight contains them all
  • Bees seeUltravioletBut not red

The seven colours

The traditional list of seven colours of the rainbow was set by Isaac Newton in the late 1600s. He saw that white light passing through a prism split into a continuous band of colour, but chose to name seven distinct colours: Red, Orange, Yellow, Green, Blue, Indigo, Violet. The first letters spell ROYGBIV, a useful memory aid. Newton picked seven partly because he was fascinated with the number seven (seven notes on a musical scale, seven planets known at the time, seven days of the week).

In reality the rainbow is a smooth blend of colours, with no sharp lines. Some modern colour scientists drop "indigo" since most people cannot easily tell it from blue. But the ROYGBIV tradition stuck.

Wavelength and colour

Each colour of light has a different wavelength: the distance from one wave peak to the next. Wavelengths in the visible range are measured in nanometres (nm), billionths of a metre.

  • Red: about 620-700 nm. Longest visible wavelength, lowest visible frequency.
  • Orange: 590-620 nm.
  • Yellow: 570-590 nm.
  • Green: 495-570 nm. Peak sensitivity of the human eye.
  • Blue: 450-495 nm.
  • Indigo: 425-450 nm.
  • Violet: 380-450 nm. Shortest visible wavelength, highest visible frequency.

How we see colour

The retina at the back of your eye contains two types of light-detecting cells:

  • Rods: very sensitive, work in low light, but only detect brightness, not colour. About 120 million per eye.
  • Cones: less sensitive, need brighter light, but distinguish colours. About 6 million per eye, in three varieties:
  • S-cones (most sensitive to blue, around 420 nm)
  • M-cones (most sensitive to green, around 530 nm)
  • L-cones (most sensitive to red, around 560 nm)

Your brain compares the signals from the three cone types to work out the colour of incoming light. Yellow, for example, excites both green and red cones. Pure white excites all three cones equally.

Fact About 8 per cent of boys and 0.5 per cent of girls have some form of colour blindness, usually red-green colour blindness. This is because the gene for red-cone production is on the X chromosome, and males have only one X. Most colour-blind people can still see colour normally; they just struggle to tell certain shades apart. Famous test designs ("Ishihara plates") use coloured dot patterns to detect colour blindness.

Mixing colours

Colours mix in different ways depending on whether you are adding light or mixing paint.

  • Additive mixing (with light): add red and green light, you get yellow. Add red, green and blue equally, you get white. Used in TV screens and computer monitors, which produce colour using tiny red, green and blue pixels.
  • Subtractive mixing (with paint or ink): paint absorbs some colours and reflects others. Yellow paint reflects red and green wavelengths, absorbs blue. The colours we see when we mix paints come from what is reflected, not added. Used in printing, with cyan, magenta and yellow inks (plus black).

The full electromagnetic spectrum

Beyond the visible range are many other kinds of light. From shortest to longest wavelength:

  • Gamma rays: highest energy, used in some cancer treatments. Produced by radioactive decay and stars.
  • X-rays: used for medical imaging.
  • Ultraviolet (UV): causes sunburn, makes white clothes glow under "black light".
  • Visible light: a tiny slice in the middle (400-700 nm).
  • Infrared (IR): felt as heat. Used in remote controls and night vision.
  • Microwaves: used in cooking and Wi-Fi.
  • Radio waves: longest wavelengths, used in broadcasting, mobile phones and radar.

All of these are the same kind of wave (electromagnetic), just at different wavelengths. The visible part we see is a tiny window in the middle.

Did you know? Many animals see colours differently from us. Bees see ultraviolet but not red. Flowers that look plain white to us often have stunning UV patterns visible to bees, guiding them to the nectar. Mantis shrimps have a staggering 16 types of cone (compared to our 3) and can see colours we cannot even imagine. Dogs and cats have only 2 cone types (yellow and blue), so they see less colour than humans.

Why the sky is blue

When sunlight (containing all colours) enters Earths atmosphere, it scatters off the air molecules. Blue light scatters about 6 times more than red, because blues shorter wavelength interacts more strongly with the tiny molecules. The scattered blue light fills the whole sky from all directions, making it look blue overhead.

At sunrise and sunset, the Suns light has to pass through much more atmosphere to reach you (sideways through it, instead of straight down). Most of the blue has been scattered away long before the light reaches your eyes, leaving the longer-wavelength reds and oranges to dominate. That is why sunrise and sunset look so warm-coloured.

Try this Fill a clear glass with water and stand it in bright sunlight on a sheet of white paper. Tilt the glass slightly until you see a rainbow appear on the paper. The water acts as a prism, splitting white sunlight into all its colours. Or, find an old CD or DVD and tilt it slowly in the light: the closely spaced tracks on the disc act as a diffraction grating, splitting reflected light into a rainbow pattern. Both are simple ways to see the visible spectrum at home.
Deeper dive: how Newton broke an ancient idea about colour

For over 2,000 years, people believed that white light was somehow "pure" and that adding things to it made it coloured. The ancient Greeks thought colour was a mix of light and dark. Even by 1660, many natural philosophers thought a prism somehow "added" colour to the originally pure white sunlight passing through it.

Isaac Newton proved otherwise with a beautifully simple experiment around 1666. He took a glass prism and let sunlight from a small hole in a window shutter fall on it. The prism split the light into the familiar rainbow band, which he called a spectrum. So far, so traditional.

Then Newton did something new. He used a second prism to recombine the spectrum into white light again. The colours all blended back into the original white. This showed that white light was a mixture of the colours, not a pure thing that the first prism somehow added to.

Newton went further. He passed individual coloured beams from the first prism through the second prism. Each coloured beam stayed the same colour. Red was always red; blue was always blue. So each individual colour was "pure" in its own right; the prism did not change the colours, it only separated them.

Newtons careful experiments transformed the understanding of light and colour. He published his work in Opticks (1704), one of the most important physics books ever written. The basic facts he established (white light as a mixture, prisms as separators, the seven traditional colours) are still taught in every primary school today.

The deeper meaning of colour (that each is a different wavelength of an electromagnetic wave) had to wait another 200 years for James Clerk Maxwells theory of electromagnetism in 1864. But Newton had laid the experimental foundation.

For more, see rainbows and what is light.