Sources and absorbers

1. Electromagnetic waves transfer energy from the place where they are emitted to the place where they are absorbed.
2. The object that emits the waves is the source, and the energy in one of its stores goes down as the waves carry energy away.
3. The object that absorbs the waves is the absorber, and the energy in one of its stores goes up.
4. The waves carry energy from the source to the absorber, but they do not carry any matter with them.
5. The source and the absorber do not need to touch, and there does not need to be any material between them.
6. Energy can therefore reach an absorber that is a very long way from its source, as sunlight reaching the Earth shows.

What happens to the energy

1. Energy carried by electromagnetic waves is transferred by radiation, which is one of the pathways by which energy moves between stores.
2. Inside the source, the energy comes from a store such as the nuclear store of the Sun, the chemical store of a fuel or the thermal store of a hot object.
3. The waves then carry the energy across the gap, at $3.0\times 10^{8}m/s$ in a vacuum or in air.
4. When the waves are absorbed, the energy usually ends up in the thermal store of the absorber, so its temperature rises.
5. Absorbed waves can also cause other changes, such as making electrons move in a metal aerial or starting chemical reactions in the cells of a leaf.
6. A wave only transfers energy to a material if the material absorbs it, so waves that pass straight through a material, or are reflected from it, leave it unchanged.
7. This is why the dark seat of a car parked in sunshine gets much hotter than the window glass, which lets most of the visible light pass straight through.

Examples across the spectrum

1. The Sun and the Earth
   1. The Sun emits infrared, visible light and ultraviolet, which cross space and are absorbed by the ground, the sea and the air.
   2. This transfer warms the Earth and supplies the energy that plants use for photosynthesis.
2. A grill or electric heater
   1. The hot element emits infrared, which is absorbed by your skin or by food, so its thermal store increases.
3. A microwave oven
   1. The oven emits microwaves, which are absorbed by the water in food, so the food heats up from the inside as well as the outside.
4. A radio transmitter
   1. The transmitter emits radio waves, and a radio's metal aerial absorbs a tiny fraction of them, transferring energy to the radio's circuit.
5. An X-ray machine
   1. The X-ray tube emits X-rays, and the bone or detector that absorbs them gains energy.
6. A radioactive source
   1. Some unstable nuclei emit gamma rays, and any body tissue that absorbs them gains energy, which is why gamma rays can damage cells.