Here is some listening practice, together with the tape script below. Click on the link to hear the audio.
Why is the sky blue?
Well, first we need to understand a little bit about how light works. When light travels from the sun, it moves up and down like a wave, similar to the waves you see on the ocean. Some waves are close together and others are further apart. The distance between each bump in a wave is called wavelength, and when all of the different wavelengths travel together, light appears white to our eyes. But if you break up sunlight so that the waves are separated, you will see individual colours. In fact, every colour has its own unique wavelength. Bluish colours have a short wavelength and move up and down more often than a colour like red, which has a longer wavelength.
So if light from the sun contains all of these different colours, why does the sky appear blue? Well, it also has to do with the way these waves interact with the atmosphere. If there were nothing between the sun and our eyes, the sun would look like a white circle in a black sky. But the atmosphere gets in the way and changes the way the light behaves. While the atmosphere may seem like a large empty space, it is actually made up of a whole bunch of tiny little particles like air molecules, water, and dust. Light waves are tiny too, so when they finally reach the atmosphere they have a hard time dodging the small particles in the sky. The shorter wavelengths of light, like blue and violet, move up and down so much they tend to interact with the particles more often than other colours. These colours get bounced around so much they spread through the atmosphere and fill the sky. Even though purple light is also scattered out across the sky, our eyes are more sensitive to blue light, and so the sky appears more of a bluish colour. Red, yellow, and green coloured lightwaves bounce around too, but not as much, and more of this light passes through. When these colours are mixed together, they appear a yellowish-white, which is why the sun looks somewhat yellow to our eyes.
So now you know when you look at a blue sky you are actually looking at a portion of sunlight that has been broken up and scattered by billions of tiny particles.