0.7 Electromagnetic radiation  (Page 2/6)

These mutually regenerating fields travel through empty space at a constant speed of $3\times {10}^8\mathrm{m}·{\mathrm{s}}^{-1}$ , represented by $c$ .

Although an electromagnetic wave can travel through empty space, it can also travel through a medium (such as water and air). When an electromagnetic wave travels through a medium, it always travels slower than it would through empty space.

Since an electromagnetic wave is still a wave, the following equation still applies:

Except that we can replace $v$ with $c$ (if we're dealing with an electromagnetic wave travelling through empty space):

Electromagnetic spectrum

Electromagnetic radiation allows us to observe the world around us. Some materials and objects emit electromagnetic radiation and some reflect the electromagnetic radiation emitted by other objects (such as the Sun, a light bulb or a fire). When electromagnetic radiation comes from an object (whether the radiation is emitted or reflected by the object) and enters the eye, we see that object. Everything you see around you either emits or reflects electromagnetic radiation or both.

Electromagnetic radiation comes in a wide range of frequencies (or wavelengths) and the frequencies of radiation the human eye is sensitive to is only a very small part of it. The collection of all possible frequencies of electromagnetic radiation is called the electromagnetic spectrum, which (for convenience) is divided into sections (such as radio, microwave, infrared, visible, ultraviolet, X-rays and gamma-rays).

The electromagnetic spectrum is continuous (has no gaps) and infinite. In practice, we can only use electromagnetic radiation with wavelengths between (very roughly) ${10}^{-14}\mathrm{m}$ (very high energy gamma rays) and ${10}^{15}\mathrm{m}$ (very long wavelength radio waves) due to technological limitations in the detectors used to receive electromagnetic radiation and in the devices used to produce or emit electromagnetic radiation.

The various frequencies (or wavelengths) of electromagnetic radiation coming from a particular object or material depends on how the object or material reflects and/or emits electromagnetic radiation.