Light is electromagnetic radiation that can be visible or not. It has properties like speed, reflection, and wave-like behavior, and can be classified by wavelength, frequency, intensity, and polarization. The speed of light is constant and important in physics. Light behaves as both a wave and a particle, with the smallest physical quantity being a photon.
The term “light” can be used to refer to visible light, which is electromagnetic radiation of a wavelength that can be perceived by the human eye, or more generally to electromagnetic radiation of any wavelength. There are many different properties of light that are shared by all electromagnetic radiation, including its speed in a vacuum, ability to be reflected, and wave-like behavior in most situations. Different properties of light can be exhibited by different light waves. Such variable properties include wavelength, frequency, intensity and polarization. The quantum mechanical properties of light are of particular interest in physics and chemistry and are based on the fact that light behaves both as a wave and as a particle.
A variety of different properties of light can be used to describe and classify a given wave of electromagnetic radiation. The wavelength of light describes the distance between two peaks in the wave or the distance between repeating sections in the wave. Frequency describes the number of repetitions that occur in a given period of time. Other properties of light, such as intensity and polarization, can also be used to classify specific light waves.
Light travels in vacuum at 186.282 miles per second (about 299,792,458 meters per second). This speed is known as the “speed of light” and is extremely important in physics for a variety of reasons, including its place in Einstein’s special theory of relativity. The theory states that “the speed of light in a vacuum is the same for all observers regardless of their motion or the motion of the light source.” Therefore, light emitted by a light source moving near the speed of light travels at the same speed as light emitted by a stationary light source. Special relativity leads to phenomena such as time dilation, length contraction, and the idea that the maximum velocity is necessarily finite.
The quantum mechanical properties of light are mostly related to the wave-particle duality, the fact that light behaves, at various times, as a wave and as a particle. Experiments have demonstrated wave-like properties of light, such as interference, polarization and diffraction. An experiment demonstrating the “photoelectric effect,” however, demonstrated that light also exhibits particle-like properties that something entirely wavelike couldn’t demonstrate. The basic “particle” of light is known as a “photon,” which is defined as a single quantum of light, or the smallest physical “quantity” of light that can exist in a single unit.
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