Emma Jukes of Palmerston North asks :-

Why do mountains appear to be blue when seen in the distance?

Henning Klank, a physicist at Massey University, responded.

Scattering of light is basically what makes the mountains appear blue in the distance. Scattering is the reason why we are able to see the light beam of a movie while sitting in the movie theatre. The same is true for the blue sky.

How do we perceive the colour of an object? Light from a light source (eg the sun) is scattered/reflected from the object and passes through the air to our eye which responds to a small range from red to violet. So the colour we see depends on what the source emits, the molecules in the object that cause the scattering/reflection, how light interacts with air, and how our eye responds to light.

Think about how the sun shines on a cloud free day. The atmosphere consists of air, which consists of very small molecules. Air seems to be very transparent over shorter distances. The light from the sun passes through our atmosphere. If we look at the sun directly (or almost directly, which would be better for our eyes) when it is high we see what appears to be white light. If we look away from the sun we see the blue sky which shows us that blue light is scattered.

The molecules of air scatter light with the shorter wavelengths (violet) scattered much more than the longer wavelengths (red). We should therefore see a violet sky, but our eyes are more receptive to blue in comparison to violet and blue is scattered almost as much as violet.

When we look at mountains in the distance then the mountains are often dark and we see blue light scattered by the air between. The more distant the mountain, the longer the air path and the bluer it appears.

If we looked directly at the sun we would see the sun in yellow light, because some blue light is scattered away on the path between the sun and our eyes. This is most dramatic at sunset, when the sun appears red, because the path through the atmosphere is rather long.