Why can't humans see infrared or ultraviolet light?

Humans can't see infrared or ultraviolet light because our eyes' photoreceptors are not sensitive to these wavelengths.

The human eye is a complex organ that allows us to perceive light and colour. It does this through specialised cells called photoreceptors, located in the retina at the back of the eye. There are two types of photoreceptors: rods, which are responsible for vision in low light, and cones, which are responsible for colour vision and detail.

The cones are further divided into three types, each sensitive to different wavelengths of light: short (S), medium (M), and long (L). These correspond to blue, green, and red light respectively. The range of wavelengths to which our eyes are sensitive is approximately 400 to 700 nanometres, which is the visible light spectrum.

Infrared light has longer wavelengths, typically around 700 nanometres to 1 millimetre, while ultraviolet light has shorter wavelengths, typically around 10 to 400 nanometres. These fall outside the range that our photoreceptors can detect, hence we cannot see them.

It's worth noting that some animals, like certain species of snakes and birds, can see in the infrared or ultraviolet spectrum. They have evolved this ability to help them hunt or navigate. For example, some birds can see ultraviolet light, which allows them to see patterns on flowers that are invisible to the human eye, helping them find nectar.

In summary, the reason humans can't see infrared or ultraviolet light is due to the specific sensitivity of our photoreceptors. They are tuned to the wavelengths of the visible light spectrum, and do not respond to the longer wavelengths of infrared light or the shorter wavelengths of ultraviolet light.