As demonstrated by the work of Young, Poisson, and Fresnel, and later by James Clerk Maxwell (1831–1879), and Heinrich Hertz (1857–1894), light is an electromagnetic wave to which human eyes respond. It is located on the electromagnetic spectrum between infrared and ultraviolet. The limits of human vision define the lower and upper boundaries of light. The lowest frequency is 4 x 10¹⁴ Hz, which has a wavelength of 700 nanometers (700 x 10^-9 m). Its upper boundary is 7.9 x 10¹⁴ Hz, a wavelength of 400 nanometers. Wavelength rather than frequency is commonly used when describing light because until the past three decades only wavelength measurements were possible—light frequencies were too high to measure directly.

Light has all the properties of a transverse wave. That is, it can transfer energy and momentum. It obeys the principle of superposition and can be diffracted and interfere with itself. On the other hand, it also has the properties of a massless particle. While in a medium it moves in a straight line at a constant velocity. It can transfer energy and momentum. A full description of the true nature of light—wave, particle, or both—will be discussed in the “What Is the World Made Of?”