To understand the nature of colours, first, we need to understand that visible light, along with ultraviolet light or radio signals among others, are types of electromagnetic waves. Just like any wave, light waves have an oscillation - moving up and down at certain rates. This is known as the frequency of the wave. Visible light isn't made up of a single frequency but is instead composed of a spectrum of waves from low-frequency red waves to high-frequency violet ones. A combination of all the waves each with a different frequency results in white light. How then can we observe this effect with the naked eye? Well, simply by exaggerating the effect of the different velocities. This can be done by passing light through different materials. Once the light enters a prism, for example, it bends through the prism differently according to the wavelength of the wave. This is known as refraction. Red light has a longer wavelength and changes direction the least, while violet light has a shorter wavelength and changes direction the most. Once the light exits the prism, the change of paths creates a visible spectrum of the different colours in the order of their velocities: red, orange, yellow, green, blue, indigo and violet. Another way to see this kind of display, in nature, is when you see a rainbow.

Rainbows use the same principle as a prism to separate colours by using the water of rain droplets. Light gets refracted as it enters the water droplet, then reflected (just like a mirror) and finally refracted again when it exits the droplet. Both the refraction and the reflection create the path difference necessary to separate the different colours. In order to see a rainbow we don’t only need sunshine and rain at the same time, but also a certain angle between the incoming sunlight and us, the observer. This needs to be between 40-42 degrees - if the angle was more or less than this, we wouldn’t be able to see it. The fact that this window is so narrow is the reason why rainbows are not as common as we might like them to be! When you’re looking at a rainbow, the light arriving at your eyes is the sunlight refracting and reflecting on specific droplets of water, which will be different to the ones refracting and reflecting the light to your friend’s eyes standing next to you. And so rainbows are not just considered striking to look at - what we see is also unique to us as individuals.  

We see objects of different colours depending on which colours are absorbed and which ones are reflected. A blue shirt, for example, reflects blue light while absorbing all the other ones. A red shirt, on the other hand, reflects only red light, while absorbing the others. However, we also observe objects with colours different to those in the colour spectrum. This happens when two or more coloured lights are reflected. For example, when both blue and red lights are reflected the shirt will look magenta (a purplish-pink colour). What happens if all colours are absorbed? Well, nothing is reflected and so the shirt looks black. This is why black is technically not a colour but rather an absence of it. On the other extreme, if all colours are reflected, then none are absorbed and the shirt looks white. Just as black was the absence of all colours, white is the combination of them all. This is also the reason why a white outfit might be a cooler choice in the heat of the summer! White clothing reflects all (visible) radiation, while black clothing absorbs it. 

At the back of the human eyes, there are photoreceptor cells called cones that work as sensors, designed to distinguish between short, medium and long wavelengths. These cones are most commonly known as blue, green and red-sensitive cones. The human brain then interprets the signal from these three sensors to work out the colour of the object. Although three types of sensors doesn't sound like much, many of us would have learnt at school that the combination of these primary colours can generate a whole range of new colours. Plus, each sensor is able to distinguish between 100 shades of each colour, allowing the brain to see more than a million colours! Colour TVs also make use of this concept. Instead of using a large quantity of colours in the display, if you look closely you’ll notice that only three are used: red, blue and green. By using different combinations of these colours, they are able to recreate colours as we see them in real-life. The same goes for printers. The only difference is that they rely upon a different trio of colours: magenta, yellow and cyan. By subtracting these colours, instead of adding them like before, we obtain the primary ones.

As mentioned earlier in our Big Question, the human brain uses the combination of red, green and blue sensors in the eye to recreate different colours. However, this process and its outcomes are altered when someone is colourblind. This is because, when someone is colourblind, one of the three types of sensors in their eyes is not functioning as they should. Subsequently, the information received by their brain is restricted which limits the number of colours they can distinguish. This situation is different again for animals and largely depends on the make up of their own visual systems. Dogs, for example, have only two cones of light and therefore are able to distinguish a smaller range of colours in comparison to us. Other animals have developed visual systems motivated by their evolutionary needs. Geckos, for example, are able to see colours in the dark, allowing them to hunt at night.  Other animals, like butterflies, have more than three sensors and are therefore able to see not only in visible but also ultraviolet light! This means they can appreciate the world on a whole new level!

The most primitive ways to see are so simple that we might not even recognise them as vision. In the very early stages of the evolution of the eye, animals were probably only able to detect whether their surroundings were light or dark. Flatworms are tiny, invertebrate (without a backbone) animals which tend to live in dark, moist environments where good eyesight isn’t that important. As a result, they never quite moved beyond this simple way of perceiving the world. So, 700 million years after they first evolved, flatworms still rely on ‘eyespots’. These are made up of a single layer of photosensitive cells which contain a pigment that detects light intensity and direction – and nothing else. This helps flatworms to move away from light – which can signal danger, from predators or the threat of drying out in the sun – and back into the safety of the dark. Amazingly, scientists have recently discovered that flatworms are still able to detect light even when they’ve had their head cut off! However, without a head, the animals only respond to UV light, meaning that this is probably an entirely separate way of perceiving the world – and possibly an even more ancient form of vision.

Image credit: Richard Ling via Wikicommons CC BY-SA 2.0

Contrary to popular belief, dogs don’t see the world in shades of grey. As mentioned earlier in our Big Question, colour vision is mediated by a special type of cell, found in the retina, called a cone. Humans have three types of cone, which detect red, green, and blue light. Dogs, like most other mammals, only have two - we’re the exception here! In dogs, these are specialised to detect yellow and blue light. So while your dog’s world isn’t exactly greyscale, it still isn’t quite as colourful as ours. Dogs’ eyesight is limited in a few other ways too. Their eyes are less sensitive to grey shades, they are half as sensitive to brightness changes as we are, and they tend to be near-sighted – meaning their vision across distances is quite poor. However, they do have a better field of view, allowing them to pick out objects over 240 degrees, versus our 180. They are also much more sensitive to motion, and so when you catch your dog watching television, they're probably seeing something more akin to flickering frames rather than continuous movement. These adaptations probably relate back to dogs’ past lives as wolves, when they would have been important adaptations for hunting. 

The most powerful eyesight in the Animal Kingdom is wielded by the eagle. With vision that is estimated 4-8 times better than ours, eagles are able to spot a rabbit from several miles away. Most birds of prey share this extraordinary vision, which is essential when keeping out of sight of prey which may be a long way below you. In most vertebrate (backboned) animals, images are formed by the retina. This tissue layer contains photoreceptive cells, which are held in a small pocket called the fovea, giving sharp central vision. The eagle retina contains many more photoreceptive cells than in humans – about one million per mm2 vs our 200,000 mm2 – while their much deeper fovea acts like a telephoto lens, magnifying objects that are very far away, and in incredible detail. And it doesn’t stop there. Like most birds, eagles are tetrachromats, meaning that they have four different types of colour-detecting cone cell, compared to our three. This means they can see a larger spectrum of colour, and so their world is much more vivid than ours. They can even detect UV light, which can allow them to detect the UV-reflecting urine trails of prey like mice.

Rattlesnake eyes are well adapted to a nocturnal life. They contain lots of light-detecting rod cells, maximising their ability to see images in the dark. They also have cone cells, but only have two types (meaning they, like dogs, are dichromats), though many are also sensitive to UV light. In species that hunt during the day, a UV filter has evolved over the eye to protect the lens. This filter is often tinted – and is the reason some snakes look like they have yellow eyes. Snakes have no fovea, so their vision is blurry and is mostly reliant on movement perception. However, some snakes (venomous pit vipers, pythons and boas) have evolved a very unusual way of detecting their prey that doesn’t depend on light: they detect infrared instead. This unique ability is achieved through a specialised hole in the face called a pit organ, which can detect infrared radiation for up to one metre. These organs use the same receptors that we use to detect the flavours of wasabi and mustard! At night, this can let them ‘see’ an image of their prey, just like an infrared camera. This vision is so good that blind rattlesnakes can use it to target vulnerable parts of their prey!

Some of the strangest eyes in the oceans belong to a group of fish called the Barreleyes or, perhaps more appropriately, spookfish. These strange-looking sea creatures appear to have four eyes, two pointing downward and the other two pointing straight up. In reality, these are two eyes split into two parts. Known as diverticular eyes, they allow the fish simultaneous vision below and above its body. Spookfish are confined to the deep sea, where very little sunlight is able to penetrate, and so their eyes are specialised to detect bioluminescence – light released from organisms – and use this to detect predators, or pick out the location of prey. The eyes don’t contain cones, so spookfish can’t see colour, but do have lots of light-detecting rod cells, maximising light detection. For a long time, we didn’t know how these odd eyes work. Then, in 2008, the first live spookfish was caught, finally allowing detailed research. Scientists found the fish have a unique way of forming an image. A mirror, rather than a lens, directs light to focus on the retina. This produces a bright, high-contrast image – perfect for picking out small flashes of light in the pitch black of the deep ocean.

Image credit: Brauer, A via Wikicommons

The mantis shrimp’s visual system is one of the most complex on the planet. These small crustaceans have compound eyes with the same hexagonal-patterned structure you’ll see on a fly. Rather than a single lens that focuses light onto the retina, these are made of numerous repeating units - each comprising a lens and some light detecting cells – called ommatidia. Each independently forms an image, which combines with its neighbours to form a coarse and grainy mosaic. The more ommtidia, the better the image – but this means that for a compound eye to be as good as ours, it would need to be a metre wide! What the mantis shrimp lacks in resolution, it more than makes up for in colour perception. Mantis shrimp eyes contain six rows of modified ommatidia, called the mid-band. Each is specialised to detect certain wavelengths of light, contain 12-16 photoreceptors (versus our three), and allow detection of ultraviolet, infrared and polarized light. Each eye is further divided into three sections, giving trinocular vision, and the ability to perceive depth with only one eye. Top this off with the ability to move each eye independently, and the mantis shrimp is really quite unusual indeed. 

Image credit: Roy L. Caldwell, University of California - National Science Foundation