Valuing our sense of sight
Like most other vertebrates, humans have complex camera like eyes. All the parts of our eyes work together to form detailed sharply focussed images. In the back of the eye we have a retina packed with two types of photoreceptors (light detecting cells): rods and cones. We have about 120 million rods. Rods are sensitive to low levels of light but they only see in greys. Our 7 million or so cones allow us to see in colour. We have three types of cones and each type is activated by different colours of light red, green and blue.
What is Light?
Visible light — the light we can see — is a tiny part of the electromagnetic spectrum. It ranges from about 390 to 770 nanometers (1 nanometer is 0.000000001 meters). Other wavelengths are invisible to us.
A common measure of electromagnetic radiation is wavelength: the distance between waves. At one end of the spectrum are very long waves - radio waves can be up to 100,000 meters long. At the opposite end are very short waves; gamma rays are less than 10 picometers (0.000000000001 meter) long.
No matter the wavelength, electromagnetic radiation travels at a constant velocity — what we call the speed of light. Shorter waves carry more energy, so they pack more punch. That's why radio waves and microwaves are harmless, ultraviolet rays can give us a sunburn, and gamma rays can kill us dead.
What is so special about visible light?
Nothing, really. Other than the fact that we can see it, visible light it is no more special than any other part of the electromagnetic spectrum. In our typical self-centred fashion, humans came up with the term 'light' to define all of the wavelengths of the electromagnetic spectrum that we can see. Humans evolved the ability to detect the wavelengths of light that are the most informative for helping us survive and reproduce in our particular environment. Similarly, other animals are able to see the wavelengths of light that are most useful to them in their environments. There are many other animals that can see wavelengths that are invisible to us — wavelengths that are both longer (infrared) and shorter (ultraviolet).
How do eyes detect light?
Light-detecting sensory cells are called photoreceptors. Photoreceptor cells come in many shapes, sizes, and configurations, but they all have something in common: they are packed full of light-sensitive proteins called opsins. When a photon (the smallest detectable unit of light) bumps into an opsin molecule, the protein absorbs its energy and temporarily changes shape. This is the first step in a signalling chain that ultimately travels to the brain. There are many types of opsin proteins. Each type has a slightly different shape and structure that makes it sensitive to light within a particular range of wavelengths. People have four types of photoreceptors: rods, and three types of cones. Rods are sensitive to low levels of light, but they do not provide information about colour. Cones allow us perceive colour. Each of our three types of cone cells is filled with slightly different opsin proteins, which are sensitive to different wavelengths of light. To perceive colour, the brain interprets the relative activity of each cone type.
How does colour vision work?
Light itself has no colour. What we call 'colour' is a subjective experience that arises from the eye's ability to tell the difference between different wavelengths of light. So while we have no way of knowing whether another person perceives colour the same way we do, we do know that most of us are able to distinguish between wavelengths.
If an animal has just one type of photoreceptor, it cannot see in colour. In order to see colour, an animal needs at least two types of photoreceptor that are sensitive to different wavelengths of light. It's the relative activation of both photoreceptor types that provides information about wavelength. The more types of photoreceptors an animal has, the more wavelengths it can tell apart.
People have four types of photoreceptors: rods, and three types of cones. Rods are sensitive to low levels of light, but they do not provide information about colour. It is cones that allow us perceive colour. Some animal eyes have physical properties that allow them to detect light and colour in other ways. Many animals have innovations that allow them to see things that we cannot.
Birds and reptiles have photoreceptors with built-in filters. Coloured pigments suspended in oil droplets inside the photoreceptors help to fine-tune each type to a different wavelength of light. The retina from a turtle, above, is shown unstained, in its natural colour. Turtles have rods and five types of cones (compared to our three) including one type for sensing quick motion. Turtles live in an especially colourful world.
Jumping spider vision
Jumping spiders only have one type of photoreceptor, but the layered arrangement of their retina gives them colour vision. Because of the properties of light and lenses, different wavelengths of light focus on different layers of the retina. Some wavelengths focus on the surface layer, and others on deeper layers. This arrangement allows jumping spiders to differentiate between about as many colours as we can.
Honey bee vision
Not only can honeybees detect ultraviolet light, they also have photoreceptors that are specialized for detecting polarized light. This ability helps them to navigate based on their position relative to the sun.
Mantis shrimp use all of these tricks and more. They have 12 to 16 types of photoreceptors, allowing them to quickly sense different wavelengths of light without the need for much interpretation by the brain. Along with having multiple types of light-sensing opsin proteins, several of their photoreceptors use filters to detect different wavelengths of light in the ultraviolet range, and some use layering to fine-tune their sensitivity. Some mantis shrimp species can even detect linear and circular polarized light —a trick that helps them see prey, which to us look totally transparent.