Blue sky

Why Is the Sky Blue?

Why Is the Sky Blue? Why Are Sunsets Red? Why Is the Ocean Blue?

In order to understand the answer to these questions, we must first understand some principles of light.  Firstly, white light is composed of all colours. It can be broken down into individual colours with the use of things like a prism.


It is a principle of nature that blue light is the most easily scattered of all the colours.  Another principle is that light is bent and scattered any time it passes through transparent materials.   This scattering is known as Rayleigh scattering. The amount that light is scattered depends on the properties of the matter, the thickness of the matter and the angle that the light is travelling.  Our Earth is surrounded by an atmosphere that scatters light quite nicely. When light from the sun passes through the Earth's atmosphere, most of the light travels straight through unaffected, but the blue end of the spectrum, which is more susceptible to scattering, is scattered all over the sky. As the blue light scatters, it bounces in all directions and anywhere you look, the scattered blue light will reach your eyes and the sky will appear blue. Without an atmosphere, our sky would not be blue, it would be black! This is why outer space is black. As soon as you leave the Earth's atmosphere, the 'sky' is black. Just as the sky would be black without an atmosphere, the sky is also black without the sun. This is why at night the sky is black.  Even though our earth's atmosphere is very much present at night, without the sunlight, there is no light to scatter and therefore the sky is black. So... in summary: Why is the sky blue?  Because the Earth's atmosphere induces a principle called Rayleigh scattering on the sun's rays which scatter the blue light throughout the sky.


Why Are Sunsets And Sunrises Red?

Another great question!  The answer to this question is just an extension of the last question.  At mid-day, the sun is a yellowish tint, but as the sun gets closer toward the horizon, the sun's rays have to pass through more atmosphere. This means that not only blue rays are scattered, but green light begins to scatter as well.  Because there is more atmosphere to pass through, blue and green waves get scattered so much that instead of the rays scattering

Why is the Sky Blue?

throughout the sky, they actually become lost and barely even make it through the atmosphere at all along the horizon.  This leaves only the red, orange and yellow end of the spectrum to see.  The closer the sun gets to the horizon, the redder the sun will appear.  Not only does the sun's colour change from yellow to orange, to red, but the sun becomes dimmer because more and more light gets lost in the atmosphere.

Why is the Ocean Blue?

When we pour ourselves a glass of water, most of us will say that the water is clear...which is correct.  Or is it?  Why is the water in our glass clear and the water in the ocean or lakes blue?  Oceans and large bodies of water are blue for a number of reasons. Here are 3 reasons that the ocean is blue: 

Why is the Sky Blue?

Reason #1: Water is an excellent reflector. Since water is so good at reflecting light, it will reflect the sky's blue colour. Thus, some of the blueness of the ocean comes from the sky's reflection.  

Reason #2: Rayleigh scatter applies to water as well. For the same reason that the sky is blue, water gets part of its blue colour from Rayleigh scattering. The greater the depth of the water, the darker and bluer the water will look...especially if you are under the water looking up towards the light.  

Reason #3: Water is actually blue! Despite the first two reasons, the most important reason that the ocean is blue is because water is actually a blue liquid!  Water is blue even without the first two reasons I have listed.  This fact is not obvious with small volumes of water, but the more water we put into the same place, the bluer it will look. It is no wonder that the earth is called “the blue planet”.  Not only does our sky look blue when we are standing on the earth, but the vast majority of our planet is covered with a blue liquid...water!

Our eyes are truly magnificent. We can appreciate them as we look at the world around us.  Although we may not always have our sight, if we take care of it properly with regular eye examinations, we will be able to see clearer for longer. To make sure you see all the wonders of the world, call us anytime for your regular eye health examination (403) 526-2020.

Written by Dr Clark Hyde,  20/20 Vision Care