Electromagnetic wave is the term used to describe a stream of light photons. They can vary greatly in energy and emitted radiation. Electromagnetic waves are the source of many forms of modern technology, from radio transmission to X-ray machines.

Electromagnetic waves are caused by the interaction between an electrical current and a magnetic field. Unlike other waves, the energy streams caused by the interplay of magnetic and electrical forces can travel through empty space. Just like an ocean wave, electromagnetic streams have peaks and troughs, caused by the forces pulling and pushing at one another. The wavelength is discovered by measuring the distance between two identical points of the wave from cycle to cycle. For instance, the distance between one crest and the next is the wavelength.

An electromagnetic wave is classified according to two main criteria: wavelength and energy. Higher-bearing energy waves, such as gamma rays, have short wavelengths. Waves with lower energy have a much longer wavelength. The scale that describes the different types of waves is called the electromagnetic spectrum. Radio waves are typically found at the bottom of the scale,while visible light sits in the middle and the high-powered, fast oscillating gamma rays top the scale.

Electromagnetic waves had been postulated for centuries but it was not until the 19th century that their existence became fully understood. James Clerk Maxwell, a physicist from Scotland, was able to create equations that explained the relationship between light and electromagnetic waves. Through his discoveries, science was finally able to grab hold of the concept and began to see the potential in manipulating the enormous streams of energy. Less than a century later, the electromagnetic wave had become a major part of human society through the invention of X-rays, microwaves, radio, and television transmission.

Whether or not an electromagnetic wave emits dangerous amounts of radiation depends on how much energy it bears. Radio waves carry very little energy and are generally considered to be perfectly safe, even when one is directly exposed to them. On the other end of the spectrum, gamma rays are the result of atomic explosions and can bear vast amounts of poisonous radiation. Exposure to high-energy waves can result in cell mutation, cancer, and possible death. However, experiments with high-level radiation are usually tightly controlled, and accidental exposure is unlikely.

Most information about outer space comes not from photos or probes but through studying the electromagnetic wave. Radio astronomers are able to create pictures of far-off space by surveying the types of waves in the vicinity. Since it is impossible for humans on Earth to see deep space, electromagnetic waves are in fact one of the best ways of understanding what is out there. The need for better information has also lead to the construction of telescopes in space, as the Earth's atmosphere prevents some information from getting through to Earth-bound equipment.