Electromagnetic radiation, or light, is a form of energy that travels through space. It has both particle- and wave-like behavior, and can be described by properties such as wavelength (the distance between successive wave crests), frequency (wave cycles per second), or energy per photon (a photon is a massless particle of light). The electromagnetic spectrum is a way to group together all types of light, such as radio waves, infrared and visible light, ultraviolet rays, x-rays, and gamma rays. At one end of the spectrum, there are radio waves—electromagnetic waves with long wavelengths, low frequencies, and lower energy. At the other end of the spectrum, there are gamma rays—electromagnetic waves with short wavelengths, high frequencies, and higher energy.

By convention, each of these categories of light is typically measured by properties and units that are convenient for that category. For example, scientists usually talk about infrared and visible light in terms of wavelengths (e.g., the human eye perceives visible light wavelengths ranging from about 400 to 700 nanometers), and radio waves are usually referred to by frequency (e.g., FM radio stations transmit on frequencies between 88 and 108 megahertz). X-rays and gamma rays are typically referred to in terms of energy (e.g., the Chandra X-Ray Observatory has instruments that study light from the universe in the energy range of 0.1 to 10 kiloelectron volts). However, these conventions are quite arbitrary. Any type of light could be described using units of wavelength, frequency, or energy; the relationships among the properties of light are the same for all categories across the electromagnetic spectrum.

The wavelength, frequency, and energy of an elecromagnetic wave are mathematically related. According to the wave-model of light, f = v/λ (frequency = speed of light/wavelength).

Light can also be described as photons. The energy of a photon is directly proportional to the frequency of the electromagnetic wave.

E = hf (energy = Planck's constant x frequency) where h is a physical constant.

The energy per photon should not be confused with intensity. Energy per photon depends only on the frequency or wavelength of the radiation. Intensity is a measure of brightness and is related to the total amount of the electromagnetic radiation; it is determined by the number of photons that pass through a given area per unit of time. Changes in intensity do not change the energy per photon. For example, a bright red laser pointer emits more photons than a dim red laser pointer, but the energy per photon for both laser pointers is the same.

Electromagnetic radiation of different energies interacts with matter differently. For example, visible light does not penetrate through walls very well, but gamma rays and radio waves are easily transmitted through them. Electromagnetic energy can be absorbed by atoms and molecules in matter; in the example above, the atoms in the walls absorb visible light, but gamma rays and radio waves pass through because they do not have the particular eneriges needed to interact with those atoms. You can think of gamma rays as being so small that they pass between the atoms, and radio waves being so large that they go around the atoms. However, whether one type of electromagnetic radiation is more or less penetrating than another is not straightforward—different kinds of materials have varying abilities to block different kinds of light.

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