Rainbow is an arch of light exhibiting the spectrum colors in their order, caused by drops of water falling through the air. When sunlight enters a drop of water, it is refracted, or bent, and reflected from the drop so that the light appears as a spectrum of colors. The colors are visible only when the angle of reflection between the sun, the drop of water, and the observer's line of vision is between 40° and 42°. Rainbows are usually seen in the sky opposite the sun following a rain shower and also in the spray of waterfalls.
Spectrum is a rainbowlike series of colors, in the order violet, blue, green, yellow, orange, and red, produced by splitting a composite light, such as white light, into colors. The rainbow is a natural spectrum produced by special weather conditions, but a similar effect can be seen when sunlight passes through a glass prism.
When a ray of light passes from one transparent material, such as air, into another, such as glass, it bends; when it reenters the first material, air, it bends again. This bending is called refraction; the amount of refraction depends on the light's wavelength.
A device for producing and observing a spectrum visually is a spectroscope; a device for observing and recording a spectrum photographically is a spectrograph; a device for measuring the brightness of portions of the spectrum is a spectrophotometer; and the science of using these devices to study spectra is called spectroscopy. Beyond the violet end of the spectrum are ultraviolet radiations, invisible to the human eye. Similarly, beyond the red end of the spectrum are infrared radiations, also invisible. The electromagnetic spectrum includes these invisible radiations, and other radiations beyond them; radio waves beyond infrared, and X rays and gamma rays beyond ultraviolet. Different parts of the spectrum have different frequencies and wavelengths, but they all travel at the speed of light, about 300,000 km per second (about 186,000 mi per second).