Most humans are trichromatic; that is, our colour vision is mediated by three types of light receptor in our eyes. These receptors are known as cones and the three types have peak sensitivity in different parts of the colour spectrum. We sometimes refer to these as LMS cones because of their peak sensitivity at long-, medium- and short-wavelengths light.
Some people (men, in the main) are colour blind and this is because they are anomalous trichromats (they have three cones but the spectral sensitivities are less optimal than they should be or they are dichromats (they are missing the L, M or S cone types). But what about other species?
Most mammals are dichromats including dogs and cats. However, many fish and birds have better colour vision than do we humans. I just came across an article that reports that chicknes have five cones compared with our three. The research has been conducted the Washington University School of Medicine in St. Louis (USA). It is suggested that birds often have more cones than we do because they descended directly from dinosaurs and never spent any part of their evolutionary past living in the dark.
We all know that Newton wrote about the colour spectrum having seven colours. As a child I wrote the mnemonic – Richard Of York Gave Battle in Vain. The sentence makes reference to Richard III, an English king who was defeated by Henry VII in 1485 at the battle of Bosworth. In order to avoid reference to this defeat, people from Yorkshire developed the alternative Rowntrees Of York Gave Best In Value, apparently. Rowntrees was a chocolate factory formed in York in 1862 (exactly one hundred years before I was born).
When I was young I naively thought that everyone would know the Richard of York line. I was surprised, later in life to learn that in the USA a different mnemonic is normally used: Roy G Biv (the name of a person). I find this American mnemonic a little bizarre to be honest; are there really people whose surname is Biv? If your surname is Biv, or you know someone called Biv, please comment below. Only recently I came across a small rhyme that goes:
Red for the rainbow, orange too.
Yellow says, ‘how do you do’.
Green is the next one, green for go.
Then comes blue and indigo.
Number seven, we must not forget, is pretty violet.
Since children learn these mnemonics from an early age most people never question whether there really are seven colours in the spectrum despite the fact that we often see the spectrum in the form of a rainbow in the sky. However, if most people look closely at the spectrum they will likely state that they can only discriminate between six bands of colour; that indigo and violet are really one colour band. So why is it that we teach children that there are seven colours in the rainbow?