SNHS Physics Blog 9: Astrophysics

Clara Jeon

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Have you ever looked up at the sky and wondered.. Why is it blue? Why can’t it be pink, or green, or purple? There is a definite reason why our sky is blue and not green.

First, the light from our sun is white, not yellow! When you go outside in broad daylight, you can see that the sun is white. Light travels in waves with different wavelengths and frequencies. White light, once it goes through a prism, can separate into many colors. This separation happens with raindrops to create a rainbow or in a prism.

The air in our atmosphere is made of mostly nitrogen, oxygen, and other particles. Some of these particles in our air are the same size as the wavelength of blue light. This causes blue light to scatter and the remaining waves to go through our retina in our eyes. With all wavelengths except for blue absorbed, we end up seeing the scattered blue light in our sky.

As all the blue light is scattered and there is no more blue light to scatter, it causes the sky to appear red and orange just like the sky changes in sunsets! In fact, as the Earth rotates farther away from the Sun, none of the wave lengths end up scattering, and we end up with a black sky at night.

This also explains why we can not see stars during the day. It is not because the Sun is too bright, it is because our atmosphere reflects blue light turning the sky blue. Once you leave our atmosphere, you will be able to see stars in the day. This threshold that allows you to see stars in the day is around 62 miles. Well, 62 miles seems pretty big right?

Actually, no! 62 miles is what the skin of the apple is to an apple as 62 miles is to the Earth. The atmosphere is much smaller than we think.