Physicists resolved the centuries-old debate on the nature of light by declaring that it is both a wave and a particle, but for most practical purposes, they treat it as a wave. The distance between wave peaks in a light beam is known as the wavelength, and the total range of possible wavelengths is known as the electromagnetic spectrum. The human eye processes visual information by perceiving only a small fraction of the spectrum.
The Colors of the Rainbow
The visible spectrum occupies a narrow range of wavelengths in the electromagnetic spectrum, which theoretically extends from 0 to infinity. Moving from the shortest wavelengths to the longest, violet light begins to be visible at about 400 nanometers (4 millionths of a meter or 1.3 millionths of a foot), and red light passes again into invisibility at about 700 nanometers. Most of the sun's energy is emitted in this range of wavelengths. Ultraviolet light, x-rays and gamma rays occur at wavelengths shorter than that of violet light, and infrared light and radio waves occur at wavelengths longer than that of red light.
Light Waves Can Bend
Light travels at a constant speed of 299,792 kilometers per second (186,282 miles per second) in a vacuum, but it slows down whenever it passes through a medium. This causes a change of direction, or bending, which is known as refraction. This is the reason why your hand appears displaced from your arm when you look at it through a glass of water. Light waves from the stars refract when they enter Earth's atmosphere, and because the atmosphere moves, the stars appear to twinkle. Refraction produces the rainbow that appears when white light from a source such as the sun passes through a glass prism. The prism effectively separates the beam into its component colors, because each wavelength bends at a different angle.
Light Spreads Out from a Point Source
Light waves spread out when they pass through an aperture with a diameter comparable in size to their wavelength, which is a phenomenon known as diffraction. The diffracted light spreads out as if the aperture were a separate light source, and when you put two such apertures next to each other and shine a single light through both of them, they produce an interference patten on a screen. This phenomenon helped convince physicists that light is a wave. You can see an example of diffraction by looking at a streetlight through a curtain. The light appears to spread in all directions as it's passing through the weave of the fabric.
How We See Visible Light
The human eye consists of a lens, called the cornea, that refracts light and focuses it on a screen in the back of the eyeball called the retina. There, sensitive light receptors called rods and cones convert the light waves into electrical signals that the brain can detect. We use tools, such as telescopes and microscopes, to increase the focus of small and faraway objects so we can see them more clearly. However, we can't see objects smaller than the wavelengths of visible light, such as viruses or dim stars, with optical devices. We use other devices, such as electron microscopes, to create images of these objects.