Electromagnetic radiation is emitted from all matter with a temperature above absolute zero, and as the temperature increases, more radiation and shorter wavelengths are emitted.Visible light is emitted from matter hotter than about 700°C, and this matter is said to be incandescent. The sun, a fire, and the ordinary lightbulb are incandescent sources of light.

The behavior of light is shown by a light ray model that uses straight lines to show the straight-line path of light. Light that interacts with matter is reflected with parallel rays, moves in random directions by diffuse reflection from points, or is absorbed, resulting in a temperature increase. Matter is opaque, reflecting light, or transparent, transmitting light.

In reflection, the incoming light, or incident ray, has the same angle as the reflected ray when measured from a perpendicular from the point of reflection, called the normal. That the two angles are equal is called the law of reflection. The law of reflection explains how a flat mirror forms a virtual image, one from which light rays do not originate. Light rays do originate from the other kind of image, a real image.

Light rays are bent, or refracted, at the boundary when passing from one transparent media to another. The amount of refraction depends on the incident angle and the index of refraction, a ratio of the speed of light in a vacuum to the speed of light in the media.When the refracted angle is 90°, total internal reflection takes place. This limit to the angle of incidence is called the critical angle, and all light rays with an incident angle at or beyond this angle are reflected internally.

Each color of light has a range of wavelengths that forms the spectrum from red to violet. A glass prism has the property of dispersion, separating a beam of white light into a spectrum. Dispersion occurs because the index of refraction is different for each range of colors, with short wavelengths refracted more than larger ones.

A wave model of light can be used to explain diffraction, interference, and polarization, all of which provide strong evidence for the wavelike nature of light. Interference occurs when light passes through two small slits or holes and produces an interference pattern of bright lines and dark zones. Polarized light vibrates in one direction only, in a plane. Light can be polarized by certain materials, by reflection, or by scattering. Polarization can only be explained by a transverse wave model.

A wave model fails to explain observations of light behaviors in the photoelectric effect and blackbody radiation. Max Planck found that he could modify the wave theory to explain blackbody radiation by assuming that vibrating molecules could only have discrete amounts, or quanta, of energy and found that the quantized energy is related to the frequency and a constant known today as Planck's constant. Albert Einstein applied Planck's quantum concept to the photoelectric effect and described a light wave in terms of quanta of energy called photons. Each photon has an energy that is related to the frequency and Planck's constant.

Today, the properties of light are explained by a model that incorporates both the wave and the particle nature of light. Light is considered to have both wave and particle properties and is not describable in terms of anything known in the everyday-sized world.