Light is energy that can travel through space. It travels in a straight line and at a very fast rate: the maximum speed of light occurs in a vacuum and is about 300,000 kilometers per second. Light is often described as electromagnetic radiation—interacting electric and magnetic fields that form electromagnetic waves. Visible light (the type of light that people can see) is just one part of a large spectrum of radiation. The electromagnetic spectrum ranges from gamma rays at the highest energy level to radio waves at the lowest energy level and includes other categories of light such as X-rays, ultraviolet and infrared light, and microwaves. Each category of the electromagnetic spectrum corresponds to a range of energies or wavelengths of light; higher-energy waves have shorter wavelengths, and lower-energy waves have longer wavelengths.

Electromagnetic waves are made of fluctuating electric and magnetic fields and, unlike other types of waves, do not need a medium to travel through. In many other ways, however, electromagnetic waves behave just like any other wave. For example, light reflects off a surface just like sound would; the angle of incidence is equal to the angle of reflection. And just like any other wave changes direction when passing from one medium into another because of a change in speed, light also refracts when crossing the boundary of two different materials. Light diffracts as well—it changes direction—as it passes through an opening or around an obstacle.

However, to say that light is a wave is too simple a statement. Light also has particle-like properties. Light can be described as a stream of particles called photons, which are discrete packets of energy that have no mass. Each photon contains a certain amount of energy; for example, gamma rays consist of high-energy photons, and radio waves consist of low-energy photons. Some behaviors of light are more easily represented by thinking of light as photons rather than waves.

Although it seems like these are two entirely different ways of viewing light, the wave and particle natures of light are related. There is a mathematical relationship between the wavelength of light and the corresponding energy of a photon. Light is neither a wave nor a particle but exhibits properties of both. This idea of having both wave- and particle-like behaviors lies at the heart of quantum mechanics and is known as "wave–particle duality."

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