How many different colours can you name off the top of your head? Ten? Twenty? Fifty? I bet that no matter how many colours you listed, it’s not even close to the number of colours your eyes can see.
Seeing and perceiving do not mean the same thing. Seeing is the process that your eyes use to collect information and send it to your brain. Perceiving is how your brain takes that information and makes sense of it.
Scientists estimate that the average human can distinguish over a million different colours. But that is not true for everyone. Some people can only see a few hundred different colours. Others can see up to 100 million!
Why is this? What exactly is colour? And how do we see it?
When light hits an object, the object reflects some of that light and absorbs the rest of it. Some objects reflect more of a certain wavelength of light than others. That’s why you see a certain colour. For example, a lemon reflects mainly yellow light. A strawberry reflects mainly red light.
Objects that absorb all wavelengths of light appear black. Objects that reflect all wavelengths of light appear white.
What happens when light hits a transparent object, like water or glass? When light travels from one medium to another, the light is not reflected like it would be on a solid object. Instead, it bends. That’s because light travels at different speeds in different mediums. This is called refraction.
When light travels through a glass prism at an angle, the different wavelengths of light are slowed down by different degrees so that each colour has a different angle of refraction. As a result, you can see all of the colours contained in white light.
But the reflection and refraction of light on an object is just one part of the story. Let’s look at what happens in our eyes and brains when we see colour.
A layer called the retina sits at the back of the human eye. Your retinas are home to two types of photoreceptor cells: rods and cones. These specialized cells convert light into signals that are sent to the brain. This allows you to see.
You have 20 times more rods than cones. Rods allow you to see in low light. Cones are 100% responsible for colour vision. Have you ever noticed how hard it is to see colour in the dark? That’s because only the rods work in low light.Scanning electron microscope image of the rods (long, skinny structures) and cones (short, wide structures) in the retina (Helga Kolb [CC BY-SA 4.0] via Wikimedia Commons).
There are three types of cones: red, green and blue. Each type respond to different wavelengths of light. Long wavelengths stimulate red cones. Short wavelengths stimulate blue cones. Medium wavelengths stimulate green cones. When different combinations of cones are activated, you see the world in colour.Colour and Refraction (2017) by Professor Dave Explains (5:28 min.).
Colour vision deficiency, often called colour blindness, occurs when one type of cone is completely missing from the retina or simply doesn’t work.
As you just learned, there are three types of cones. That means there are also three types of colour blindness. The type depends on which type of cone that is missing or not working.
The loss of red cones is called protanopia. The loss of green cones is called deuteranopia. We usually refer to both of these conditions as “red-green” colour blindness. They make it very difficult to distinguish between shades of red, yellow, orange and green. This is the most common type of colour blindness.
Did you know?
The genes that make cones are on the X chromosome. This explains why 8% of men have red-green colour blindness, while less than 1% of women do.
A person with protanopia is less sensitive to red light. Remember that rainbow you saw earlier on? A person with protanopia might see it as yellows and blues, like this:A rainbow, as seen by a person with protanopia (Public domain image on Wikimedia Commons).
People with deuteranopia are less sensitive to green light. They’ll also see the rainbow as yellows and blues. However, the colours will be different. A person with deuteranopia might see the rainbow like this:A rainbow, as seen by a person with deuteranopia (Public domain image on Wikimedia Commons).
Tritanopia is a form of colour blindness where a person can’t distinguish between yellows and blues. It’s also called “blue-yellow” colour blindness. It’s a very rare condition that results from the loss of blue cones. People with this condition have difficulty distinguishing blue from green and yellow from purple. A person with tritanopia might see the rainbow as shades of red, pink and green.A rainbow, as seen by a person with tritanopia (Public domain image on Wikimedia Commons).
Another rare form of colour blindness is called achromatopsia.
Incomplete achromatopsia involves the loss of two out of the three cone types. Since the brain needs to compare signals from at least two different cones to properly identify colours, people with this condition have severely limited colour vision.
Complete achromatopsia is the loss of all three cone types. People with complete achromatopsia see the world entirely in shades of grey.A rainbow, as seen by a person with complete achromatopsia (Let’s Talk Science).
Did you know?
People with typical vision are called trichromats. That’s because their eyes have three types of functional cone cells. People with only two functioning types of cones are called dichromats.
Most types of colour blindness are the result of genetic mutations. Some mutations cause cone cells to only partially work. This leads to a milder form of colour blindness. Other mutations cause missing cones cells. Colour blindness can also be the result of brain damage, chronic illness or taking certain medications.
Did you know?
Many mammals, including nocturnal mammals, marine mammals and most New World monkeys, are dichromatic.
On the other end of the spectrum, researchers recently discovered that up to 12% of women may actually have four types of cones in their retinas! This is called tetrachromacy. A person with tetrachromacy is called a tetrachromat. Scientists have suggested that these women may be capable of seeing up to 100 million different colours! This includes colours that the average person can’t even imagine!
Peacock mantis shrimp (Odontodactylus scyllarus) with close-up of eyes (Jens Petersen [CC BY-SA 3.0] via Wikimedia Commons).
Did you know?
The prize for superior colour vision has to go to an animal called the mantis shrimp. It has sixteen different types of photoreceptors!