Radiation - heat rays
If you throw a pebble into a puddle you will make a splash. You will also see something else. Rings of waves, called ripples, will spread out from the place where your pebble hit the water. The waves that bob up and down on the water surface carry energy across it. About three hundred years ago scientists made investigations which showed that light sometimes behaved like the waves moving across the surface of a puddle. Since then they have discovered that heat also travels as waves. Light waves and heat waves have something in common. They do not need water to move in. In fact, they do not need anything to move in. They can simply move on their own.

Most of the heat on the Earth comes from the Sun. Inside the Sun changes take place, which release huge amounts of energy. When this energy reaches the surface of the Sun, much of it changes into waves of light and heat. These waves travel across space in rays. Most of space is empty but this does not affect heat waves. Heat cannot pass through space by either conduction or convection because both these forms of transport need particles. In space there are very few particles and in some places none at all. When light and heat waves reach the Earth's atmosphere they keep moving and pass through it too. Some of the light waves scatter to make the sky blue but most reach the ground. When heat waves reach the ground they are absorbed just like water in a sponge. The heat waves make the ground warm or warm the surface of lakes and seas. Heat waves warm everything they touch including you.

The movement of heat energy by waves does not stop when the Sun's rays reach the Earth. Everything on the Earth can produce heat rays. They are not as strong as the Sun and you need special equipment to detect them but the rays are there moving out into the air again and even back into space.

We had a heat wave last year. Is that the same as heat waves travelling through space?
No. The common use of the term "heat wave" means a period of hot, dry weather. Heat waves are tiny, invisible vibrations, which can pass through space and the air.

Are there any other kinds of waves besides light and heat waves?
Yes. There are radio waves, ultraviolet waves and microwaves. Two kinds of waves are called rays. They are X-rays and gamma rays. The proper name for heat waves is infrared waves.

Stars give out light. Do they give out heat too?
Yes, they do. The Sun is a star and it gives out large amounts of heat. Other stars do the same. We cannot feel their heat because as heat waves move from a star they spread out in all directions. Eventually they are so spread out that they lose their power to heat up anything they touch. We are lucky that the Earth is just the right distance from the Sun to provide enough heat for us to live. If the Earth were much closer, it would be too hot and if it were much further away it would be too cold. Now scientists are looking for planets around other stars. They are looking for planets that are just the right distance from their star to be warm enough for life.

Is radiation used to cook food?
Yes, it is. A toaster works by heating bread by radiation. The wires carrying electricity in the toaster release heat rays when the current is switched on. These rays warm up the bread and turn it into toast. A grill cooks food by radiation and heat rays rise from charcoal on a barbecue to cook the food on the grill above them.

How do the heat rays warm up a substance?
All substances are made from tiny particles, which can only be seen by very powerful microscopes. When the heat rays are absorbed by a substance the energy in them passes to the particles in the substance. This makes the particles shake about and the substance warms up.

Light is used to make a picture in a camera. Can heat be used too?
Yes. Some cameras called thermal-imaging cameras can make pictures from the heat rays they receive. Some of these cameras are used on police helicopters to find criminals who are hiding in the countryside. They are also used to take pictures of homes and show how much heat is escaping. These pictures are coloured. Some colours show that a lot of heat is escaping while others show areas where little heat is passing out into the air.

Why are clear nights colder than cloudy nights?
This is due to the amount of heat the earth radiates back into space. On a clear night the heat rays pass straight through the atmosphere to space and it feels cold. Clouds prevent a lot of heat escaping into space by radiation and it feels warmer.