Welcome to The Sound of Science on WNIJ.
(K) We’re answering listener questions this month and today we have another summer question. Andy asks, “Why do things look wavy when looking at them across the roadway in the summer?”
(P) Great question, Andy. There’s a lot of science in the answer. First, you likely know from experience that the air you are looking through when things look wavy is hotter than the air around it. That’s usually from heat radiating off a dark surface like the road or the hood of your car. Let’s talk about what that does to light speed.
You may have heard that light travels at a constant speed, which is true in empty space. As it passes through matter, including air and other transparent materials, it interacts with the electrons and nuclei of the atoms, slowing it down. Each solid material has a unique effect on the speed of light; we measure that as the index of refraction. Refraction is another word for bending light.
(K) What’s that got to do with things looking wavy when looking through hot air?
(P) We’re getting there. Let’s look at air. It’s a gas, so its density depends on temperature as well as pressure. The hotter the air, the less dense it is. Therefore, as it gets hotter there are fewer molecules per cubic centimeter. Fewer particles for the light to interact with as it passes through the hot air means light travels faster through hot air than through denser, cold air. So, the index of refraction of air depends on temperature.
Light bends when it goes from a medium to another medium with a different index of refraction. That’s how prisms work or…
(K) Or why my straw looks bent at the surface of the water in my glass.
(P) Yes, so as light goes through air of different temperatures, it’s going through media of different indices of refraction and is bent.
(K) So, Andy, the next time you see that wavy effect when looking over the hood of your car, think of the bumpy ride that light took to reach your eyes.
(P) Keep your questions coming by emailing them to firstname.lastname@example.org.
(K) This is the Sound of Science on WNIJ, (P) where you learn something new every day.