Why Is the Sky Blue and a Sunset Red? The Science of Seeing Colour
Have you ever looked up on a clear day and wondered why the sky is blue, yet a few hours later watched that same sky turn spectacular shades of red, orange and pink?
The answer involves sunlight, our atmosphere — and ultimately how our eyes and brain see colour.
Welcome to The Science of Seeing, a new series from Parkhurst and Co exploring some of the fascinating things our eyes allow us to experience every day.
And we're starting with one of the simplest-sounding questions of all:
Why is the sky blue and a sunset red?
Why Is the Sky Blue and Sunset Red?
Although sunlight looks white, it actually contains a range of different wavelengths of visible light.
These are the colours we recognise from a rainbow — red, orange, yellow, green, blue and violet.
When sunlight enters the Earth's atmosphere, it interacts with tiny molecules in the air.
Shorter wavelengths of visible light are scattered more strongly than longer wavelengths. Blue light is therefore scattered across the sky in many different directions.
When you look upwards on a clear day, some of that scattered blue light is reaching your eyes from all across the sky.
That's why the sky appears blue.
But What About Violet?
Here's where it gets even more interesting.
Violet light has an even shorter wavelength than blue and is scattered very strongly too.
So why isn't the sky violet?
There are several reasons, including the spectrum of sunlight reaching us and the way our visual system responds to different wavelengths.
Our eyes are less sensitive to violet than to blue, and the combination of light reaching our eyes and the way our brain interprets those signals contributes to the sky appearing predominantly blue.
In other words, the answer isn't just about what's happening in the atmosphere.
Our eyes are part of the story too.
Why Does a Sunset Look Red?
At sunset, sunlight takes a much longer path through the Earth's atmosphere before reaching your eyes.
During that longer journey, much of the shorter-wavelength blue and violet light is scattered away from your direct line of sight.
More of the longer wavelengths — particularly reds and oranges — remain in the light travelling directly towards you.
The result can be the spectacular reds, oranges, yellows and pinks we associate with sunset.
Dust, moisture and other particles in the atmosphere can influence exactly how dramatic those colours appear.
So the blue daytime sky and a fiery red sunset are actually produced by the same basic phenomenon — we're simply seeing sunlight after it has travelled through different amounts of atmosphere.
But How Do Our Eyes Actually See Colour?
This is where the science of seeing colour becomes particularly fascinating.
At the back of your eye is the retina, a layer of light-sensitive tissue containing specialised photoreceptor cells.
Two important types are rods and cones.
Rods are extremely useful in low-light conditions but don't provide our normal colour vision.
Cones allow us to see colour and fine detail.
Humans typically have three types of cone photoreceptor, with different sensitivities across the visible spectrum. Your brain compares the signals coming from these cones and uses that information to create the enormous range of colours you experience.
So there isn't a tiny detector in your eye labelled "blue".
Colour is an interpretation created by your visual system from patterns of light reaching your eyes.
Do We All See Exactly the Same Blue?
Probably not in an absolutely identical way.
Our visual systems are remarkably similar, but there are individual differences in colour perception.
People with inherited colour vision deficiencies, for example, may distinguish certain colours differently because one type of cone photoreceptor is absent or responds differently.
Colour perception can also be influenced by:
• Lighting conditions
• Surrounding colours
• Age
• Changes within the eye
• Certain eye conditions
• The lenses or filters we're looking through
This is one reason colour is much more complicated than it first appears.
Can Our Colour Vision Change as We Get Older?
Yes.
The natural crystalline lens inside the eye gradually changes as we age. It can become less transparent and increasingly yellow.
With the development of a cataract, colours may appear less bright or vivid, and distinguishing certain shades can become more difficult.
People sometimes don't realise how much their colour perception has gradually changed because it happens so slowly.
Following successful cataract surgery, some people are surprised by how much brighter or more vivid colours appear.
Why Do Sunglasses Change the Colours We See?
Put on a pair of sunglasses and you're deliberately placing a filter between the outside world and your eyes.
Different lens colours and tints transmit different wavelengths of light, which is why the world can look subtly different through grey, brown, green or other tinted lenses.
Some specialist lens tints are also designed for particular environments or activities where controlling glare or altering contrast may be useful.
We'll explore the fascinating science behind sunglasses, tints and colour in a future article.
Your Eyes Don't Work Alone
Perhaps the most fascinating part of all this is that seeing isn't something your eyes do by themselves.
Your eyes collect information carried by light.
Your retina converts that light into electrical signals
Those signals travel along the optic nerves.
And your brain processes and interprets that information to create the visual world you experience.
That brilliant blue sky isn't simply a photograph being projected into your head.
It's the result of sunlight, physics, your eyes and your brain all working together.
The Science of Seeing
At Parkhurst and Co Styling Opticians, we spend much of our time talking about eye health, eye examinations, glasses and lenses.
But vision is far more fascinating than simply whether you can read the bottom line of an eye chart.
In our new Science of Seeing series, we'll occasionally step away from the usual eye-care topics and explore some of the everyday mysteries of vision.
Coming up, we'll look at questions such as:
• Why can you sometimes see better out of the corner of your eye in the dark?
• Why do your eyes take time to adjust when you walk into a dark room?
• Why do optical illusions fool us?
• Why do we have a blind spot that we normally never notice?
• Do we all experience colour in exactly the same way?
Because the more you learn about vision, the more remarkable it becomes.
So next time you see a beautiful blue sky turn red at sunset, remember — you're not just looking at something beautiful. You're watching physics, your eyes and your brain working together.
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