Question: What exactly is 'colour'?

Andrew McKinley answered on 10 Nov 2014:

Now that is a very interesting philosophical question!

Light, basically, is electromagnetic (EM) radiation, just like a radio wave. The *only* difference between “light” and a “radio wave” is the energy of the wave. We call electromagnetic waves “Photons”

Depending on their energy, photons will initiate changes in molecules – very low energy photons will make molecules move, slightly higher energy photons will make molecules spin. Higher energy photons again will make molecules vibrate, and much higher energy photons will make the electrons in the molecule jump between energy levels. Every single one of these changes involves the molecule absorbing the energy of the photon.

Molecules in our eyes have electrons that are ready to jump between energy levels; when they are stimulated by a photon of the correct energy, the electron jumps. This initiates a neuron pulse to our brain. It just so happens that there are three types of ‘sensor’ in our eyes, each of which has a particular molecule sensitive to a photon of a particular energy; one molecule has electrons sensitive to lower energy photons (that we call “Red light”), another has electrons sensitive to slightly higher energy photons (that we call “Green light”), and the third has electrons sensitive to higher energy photons again (that we call, you guessed it, “Blue light”).

Our brain then receives three signals; one from each of the sensors, Red, Green and Blue. It is our brain that ‘mixes’ the signals together and interprets colour.

How then do we see “yellow”? Well, there is some ‘overlap’ between the sensitivities of each of the sensors – each can be stimulated by a range of photons, so a type of photon, such as the one we call “Yellow”, can stimulate both the Red and the Green sensors (though not as strongly as either a “Red” or “Green” photon); our brain then interprets this mixed signal as ‘Yellow’.

This leads onto another question: is the “Yellow” that I see the same as the “Yellow” that someone else sees? The answer there is not straight forward, and unfortunately I am not able to provide an answer to that one!