Atmospheric refraction

The idea

Hotter air is less dense and has a slightly lower refractive index than cooler air; as the air's physical conditions keep changing, objects seen through it appear to waver, an effect of atmospheric refraction.

Over a fire or a radiator

Look at objects through the air rising above a fire or a radiator. They seem to waver and flicker.

Why do they waver?

• The air they pass through keeps changing
• The objects themselves move
• The fire makes the objects bigger

Hotter or cooler air

Tap each statement, then tap the air it describes.

Wavering

The air above a fire is hotter, lighter and has a slightly lower refractive index than the cooler air above it. The air is not still, so the apparent position of an object seen through it changes. This wavering is atmospheric refraction on a small scale.

Hotter air is lighter (less dense) and has a slightly lower refractive index than cooler air. As the air is not stationary, the apparent position of an object seen through it fluctuates: atmospheric refraction.

Notes

Hotter air is lighter and has a slightly lower refractive index than cooler air; as the air keeps changing, objects seen through it waver: atmospheric refraction.

Check yourself

Which air is lighter (less dense)?

Compared with cooler air, the refractive index of hotter air is…

Objects seen through the hot air rising above a fire appear to waver because…

Refraction of light by the earth's atmosphere is called…

• The cooler air. The hotter air is lighter (less dense) than the cooler air above it.
• The hotter air — correct. Yes!
• They are equally dense. The hotter air is lighter than the cooler air above it.
• slightly more. The hotter air has a refractive index slightly less than that of the cooler air.
• exactly the same. The refractive index of hotter air is slightly less than that of cooler air.
• slightly less — correct. Yes!
• the conditions of the air keep changing, so the apparent position changes — correct. Yes!
• the objects themselves are moving. The physical conditions of the air are not stationary, so the apparent position of the object fluctuates.
• the hot air is perfectly still. The air is not stationary, which makes the apparent position of the object fluctuate.
• dispersion. Dispersion is the splitting of light into its colours.
• atmospheric refraction — correct. Yes!
• scattering. Scattering is a different effect. The refraction of light by the earth's atmosphere is atmospheric refraction.