Radiation

Pema and Palden felt warm sitting round the fire without touching it. Their grandfather explains that the heat comes directly from the fire to them, by a process called radiation. The heat of the Sun reaches us in the same way, across millions of kilometres of empty space.

That is the key difference: radiation needs no medium. Conduction and convection both need a material whose particles carry the heat. Radiation does not; it can cross empty space.

All objects radiate heat. A hot utensil taken off the flame cools down after a while, because it is radiating heat to its surroundings.

This explains what we wear. Light-coloured clothes reflect most of the heat that falls on them, so they feel more comfortable in summer. Dark surfaces absorb more heat, so dark clothes feel better in winter.

In everyday life all three often happen together. Put a pan of water on a flame. Heat passes from the flame into the metal pan by conduction. The water inside is heated throughout by convection. And the warmth you feel standing near the flame and the hot pan is radiation.

The book's summary:

• Conduction: a particle receives heat and passes it to the next one touching it; the particles do not move away.
• Convection: heat travels with the particles as they actually move.
• Both of these need a medium.
• Radiation: heat travels from one place to another with no material medium at all.