Ask A Scientist
Why do you see waves in the air when it is hot or coming off a hot object?
Asked by: Ruby Magura
School: Johnson City Intermediate School
Teacher: Sharon Bieber
Answer from Katie Simons and Stephen Levy
Undergraduate student/Assistant Professor of Physics
Hobbies: Reading, baking, playing board games
The waves that seem to appear near hot objects, like a campfire on a cool night, result from a phenomenon called refraction. Refraction happens when light that is passing through one substance, like air, changes its speed when it enters another substance, like glass or water. Although it is commonly said that light moves at one constant speed, this is not true. The speed of light through a substance depends on certain physical properties of the substance.
The change in speed as light crosses the boundary from one substance to another will also cause the light to slightly change its direction -- the light bends. This changing of direction is why sunlight passing through a glass prism will create a rainbow. The sunlight is made up of the combination of all the rainbow’s colors. When the light passes through the prism, each color moves at a different speed and bents a slightly different amount.
An example of a physical property that can change the speed at which light travels is density. If an object is denser, then the light slows down. For example, try running under water and compare it to running in air. It is more difficult to run under water. This is somewhat similar to the way light behaves, because it slows down in denser materials.
The density of air depends on its temperature. Cool air is denser than warm air, so light does not move as quickly through it. As light passes from warm air into cool air, the density increases and the light slows down and shifts direction. The amount of this change in direction depends upon the temperature difference.
A hot object, like a campfire, will warm the surrounding air. As the air around an object warms, it rises and mixes with the cool air above it. Light traveling towards you over the hot object would pass from cool air, through the currents of rising warm air, and then back into cool air. At each boundary, the light will change its direction. The temperature difference is constantly changing and the boundaries between the warm and cold air are moving as the heat rises, so the light rays don’t always bend the same amount or in the same place. The visual effect of this unpredictable bending looks like waves coming off of the hot object. This is the same reason that stars appear to twinkle as their light passes through the Earth’s air to where you stand on the ground.
It is not easy to fully explain why light changes its speed in a substance, but it results from the fact that all things are made of atoms. Atoms contain charged particles, called electrons, which vibrate in response to light. Interestingly, these vibrations also cause the electrons to emit light. The light that travels through the substance is then a combination of the original light and the light emitted by the electrons.