Light (Level 4)

Tupper Lake Morning (Frosty Autumn)
This image of a beautiful clear blue sky was taken on a cold autumn morning with the sun behind the camera. Red light from the sun keeps going straight away from the camera (you can see some bouncing back off the clouds) and the deep blue sky color is caused by the blue light that cannot pass straight through the sky and instead gets scattered back to the photographer's viewpoint. You can also see this clear blue color in the water. Most of that is caused by the surface reflection of the blue sky, but natural bodies of water often appear blue on their own. In such cases we see the blue color because red light is absorbed by the water and blue can pass through and reflect off objects in the water back to our eyes.

Why is the Sky Blue?

The direct answer is that the sky is blue because that is how we perceive it. Remember, color is a perception and not directly a property of objects. However, the way objects interact with light provide much of the information our visual systems use to determine what color we perceive. Thus, there is something about the sky that makes the light that reaches our eye appear blue in most circumstances.

That something is the fact that the light we see in the sky has not come to us directly from the sun, but it has been scattered by gasses and particles in the Earth's atmosphere. (Consider that in space the "sky" is black because there are no gasses or particles to scatter light and astronauts can only see light directly from the sun and objects that reflect the sun's light.) The kind of scattering that produces the blue sky is called Rayleigh scattering. That is named after a British scientist, Lord Rayleigh (his actual name was John William Strutt), who is considered the first scientist to describe this type of light scattering.

Rayleigh scattering has the property that, for particles of the size typically found in the clear sky, blue light will scatter much more than red light. When we look at the sky away from the sun, we can only see scattered light (light that has bounced around the atmosphere and not passed straight through) and since Lord Rayleigh figured out and explained that blue light will be scattered the most in the atmosphere, it is blue that we see when we look at the sky. It is for this same reason that sunsets appear red. In the case of sunsets, we are seeing the light that passes straight through the atmosphere and not the scattered blue light. Clouds look white (or gray when little light passes through them) because the condensed water or ice particles in clouds are much larger than the wavelength of light and therefore they scatter all colors equally.

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Updated: Apr. 19, 2011