T3
Colour

Colour #2: The Electromagnetic Spectrum

Visible
‹ InfraredUltraviolet ›

Colour starts as light

Our senses are our way of perceiving the world around us, each detecting something else in it. Smell and taste detect big organic compounds, your ears detect vibrations in the air.

Your eyes detect photons, the most fundamental particles in the universe, which are what light is made of! Photons are the only things that can travel at light speed (hence the name), and they have zero mass. Fascinating things!

The sun happens to emit a literal bajillion of them every second, and they love bouncing off of things, which is why many animals have evolved eyes to catch them.

There are many kinds of light

Photons are waves, and how fast the wave oscillates is called its frequency (measured in Terahertz, THz). The higher the frequency, the more energy each photon has, and the shorter the wavelength (measured in nanometers, nm), which is the distance between two peaks of the wave.

Our eyes can only detect a very small range of the electromagnetic spectrum, which we call the visible spectrum. The rest is invisible to us, but it is still light! Microwaving food, listening to the radio, getting an X-ray, using Wi-Fi — it all uses light.

You might have heard stories of people’s organs literally melting after being exposed to radioactive gamma rays, because the photons are so high in energy that they are ripping your cells apart — it’s safe to say that evolution hasn’t equipped us to deal with that part of the spectrum. However, it’s probably good that our eyes can’t detect the low-energy radio waves, otherwise we’d be constantly blinded by the Wi-Fi signals in our homes.

The spectral rainbow

The visible spectrum contains all the colours of the rainbow. You may remember the mnemonic ROYGBIV: Red, Orange, Yellow, Green, Blue, Indigo, Violet.

Eyes are basically wavelength detectors: When a 550nm photon hits your retina, your brain will see green. Make it 700nm and your brain will see red.

It varies very slightly, but the visible range is typically considered to be 380 to 780 nanometers; at least according to the International Commission on Illumination (CIE), which defines the “CIE standard observer”, a hypothetical person with perfectly average eyes.

Where is pink?

Notice how pink is not part of the visible spectrum, or any rainbow. Why is that?

It’s not because god forgot to put it there — it turns out pink is mostly a fabrication of the brain. I mean, so are all the colours, but perhaps pink is a bit more fabricated than the others.

Let’s look at how our eyes detect light in more detail, so you see what I mean.