As the Sun sends its rays to Earth, it lights up our world. But there’s more to the world than the things we can see with light alone.
As light races through space, it makes electricity and magnetism ripple down its path like waves on the sea. Light is not the only energy that behaves like this: there's a whole collection of similar waves called the electromagnetic spectrum.
Some of these waves are very long, with large spaces from one peak to the next. Others are extremely short and close to each other. Different waves have different uses, depending on their length. Although we can't see most electromagnetic waves, they are incredibly useful, helping us with everything from spotting broken bones to watching TV shows.
Light is made when atoms flash on and off like fireflies. If you heat an iron bar, it glows red hot. The atoms inside absorb heat energy, but it makes them unstable. To return to normal, they have to get rid of the energy again, and do so by giving off a flash of light. Other kinds of electromagnetic waves are made in the same way.
Energy in: When an atom gains energy, its electrons store it as potential energy by moving away from the nucleus. We say the atom is excited.
Energy out: The excited atom is now unstable. It returns to its original state by giving off the same amount of energy it stored (as light or other electromagnetic radiation).
The longest waves on the electromagnetic spectrum are radio waves, which can be thousands of miles long from the top of one wave to the next. At the other end of the spectrum, gamma rays are even smaller than atoms, and are packed full of energy. The shortest waves have the most energy and the highest frequency (meaning they vibrate the fastest), while the longest waves vibrate more slowly and have less energy.
As you travel along the spectrum you can see how different wavelengths are used for a variety of useful tasks.
Radio waves carry TV (as well as radio) signals between giant antennas. Long (AM) radio waves bounce off part of the Earth’s atmosphere called the ionosphere, especially at night, which is why you can pick up more distant radio stations in the evening.
One microwave is typically about as long as a pen. Like other electromagnetic waves, they race along at the speed of light, which makes them perfect for carrying phone calls and Internet data.
We feel the heat that things give off when the atoms in our bodies absorb a kind of hot light called infrared radiation. Although invisible, infrared radiation shows up on thermal (heat-sensitive) cameras.
The light that looks white to our eyes is really a mixture of different colors. We can see this by firing a light beam through a wedge-shaped piece of glass called a prism, which causes light to spread into the spectrum.
Rainbows work in exactly the same way. As sunlight shines through rain, each water droplet acts like a miniature prism.
Shorten blue light waves and you get energetic radiation called ultraviolet (UV). Sunlight contains two kinds of ultraviolet: UV-A and the more harmful UV-B. Small amounts of UV give a nice suntan; in bigger doses, it ages skin and causes cancer.
These short waves have enough energy to pass through soft body tissue (skin and muscle) but not bone. That's why an X-ray photo shows bones as shadows. High doses of X-rays can be very harmful.
The smallest electromagnetic waves are like super-energetic X-rays, but do much more damage to the human body. They’re made when atoms split apart in nuclear explosions.