# How Do We See Colour?

How does the human eye see visible light as colour? And why do some people see more colours than others?

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?

### What Is Colour?

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.

### How Do 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.

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.

### What Is Colour Blindness?

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:

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:

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.

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.

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.

### What Causes Colour Blindness?

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.

### Can Some People See Even More Colours?

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!

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!

• Have you ever disagreed with someone over the colour of an object? Explain.
• Do you know anyone who has a form of colour blindness? Is that person male or female? What colours do they mix up or have trouble identifying?
• Should the public fund research and development into finding a cure for colour blindness? Why/why not?
• How is colour used to help make our workplaces and daily life safe? Why are most signs of importance (e.g., stop signs, caution signs, exit sign, etc.) limited to two highly contrasting colours? Explain.
• What anatomical structures allow us to see? How do humans see light and colour?
• What are photoreceptor cells? What is the difference between rods and cones?
• What can cause red-green colour blindness?
• What combination of cones do people with typical colour vision have? What condition would cause someone to only see in shades of grey?
• If you designed an experiment where the shade of a particular colour was the dependent (responding) variable, what could you do to make sure different experimenters “saw” the same colour? Explain.
• There is no cure for colour blindness. But there are glasses for correcting certain forms of colour blindness. Conduct online research to find out more. Evaluate the pros & cons of this technology.
• Colour is often used to help market products. Given how many people have some form of colour blindness, will colour influence all people the same way? What other visual information could be used in addition to colour?
• This article and embedded video can be used to support teaching and learning of Physics, Biology and Anatomy related to visible light, vision, reflection & refraction and the human eye. Concepts introduced include shades, colours, reflects, absorbs, wavelengths, retina, rods, cones, colour blindness, protanopia, deuteranopia, tritanopia, achromatopsia, incomplete achromatopsia, complete achromatopsia and genetic mutation.
• To introduce this topic teachers could engage students in a short discussion on “What is colour?”, “How do we see colour?”, “How do our eyes work?”, etc.
• After students have watched the embedded video Colour and Refraction, teachers could follow up with a Question-Answer Relationship learning strategy to get students thinking deeper about the content and to consolidate what they have learned. A ready-to-use reproducible for this video is available to download in [Google doc] and [PDF] formats.
• Teachers could conclude by having students complete an Exit Slip to allow them to demonstrate their understanding of this topic. A ready-to-use reproducible is available for download in [Google doc] and [PDF] formats. Teachers could collect these slips and use as part of their assessment and/or to identify topics/issues that require follow-up in the next lesson.

Article by J.L. Morton for Color Matters providing facts about colour blindness, examples of what colours people with the different types may see, and examples of colour-blindness tests.

Video (1:36 min.) from Science Magazine explaining how mantis shrimps’ colour receptors work. The video says that mantis shrimp have twelve receptors. However, other sources say they may have up to sixteen.

## References

Achromatopsia Info. (n.d.). Colorblindness and achromatopsia.

American Optometric Association. (n.d.). How your eyes work.

Greenwood, V. (2012, June 18). The humans with super human vision. Discover.

National Eye Institute. (2015, February). Facts about color blindness.

Rochester Institute of Technology. (n.d.). Rods & cones.

US National Library of Medicine. (2015, January). Color vision deficiency.

Waggoner, Terrance L. (2017). What is colorblindness and the different types?  Color Vision Testing.

Wise Geek. (n.d.). What is deuteranopia?