WHY CAN’T HUMANS SEE ALL LIGHT?
The light that we see is just a small part of the electromagnetic spectrum, which stretches from radio through to X-rays and gamma rays. A fundamental property of photons is the amount of energy they carry, corresponding to the wavelength of the light (the distance for a wave to return to the same position in its cycle). For visible light, we call this the light’s colour.
No means of detecting light can ‘see’ the entire spectrum. You can’t pick up X-rays with a radio receiver, for instance. However, some animal eyes can cope with energies a little lower than humans can see – the longer wavelength infrared light – while others pick up the slightly higher energy ultraviolet light with its shorter wavelength. This can be useful because many flowers have ultraviolet patterns visible to pollinating insects, while hawks can detect small mammals in the grass far below them just from the ultraviolet glow of urine trails.
The precise colours our eyes detect depend on light-sensitive cells known as cones. We have three types of these that are most sensitive to colours around blue, green and yellow/red. The hawks seeing ultraviolet have a fourth cone type, while many mammals only have two kinds of cone, as their eyes are dominated by colour-insensitive rod cells, essential for night vision. This is because mammals were originally nocturnal. Primates like humans appear to have gained the extra cone type around 35 million years ago – our colour range, it is suggested, was particularly suited to finding fruit.