Color Spectrum
The spectrum of sunlight is a familiar sight, particularly after a rainstorm; tiny particles of water suspended in the air will split the Sun's white light into a rainbow. Anyone who has played with a prism has probably duplicated this phenomenon. The first person to examine the color spectrum in this way was the renowned English physicist and mathematician Isaac Newton, and it was he who provided the first insight into the composite nature of light.
Newton performed his optical experiments between 1665 and 1666, and they are described in detail in letters compiled and published in 1672 under the title Philosophical Transactions. Though his previous work had primarily been in mathematics and the development of calculus, he was apparently inspired by the research of Robert Boyle and Johannes Kepler to examine the nature of light. Newton obtained a beam of light by allowing sunlight to pass through a small hole in a curtain. He then directed that beam through a prism and onto a screen. Expecting a broad circle of white light, Newton was quite surprised to see instead a narrow band of colors.
At first, Newton assumed that this phenomenon was a result of the prism itself; however, when the spectrum was passed through a second prism it was recombined into a beam of white light. Also, Newton found that light of a single color could not be further dispersed--that is, a beam of red light passed through a prism would not be split into a " red spectrum."
Newton correctly presumed that the spectrum was a property of the light itself. White light, being a composite, is made up of many different colors. As it enters the prism it is bent slightly; more importantly, each color is bent to a different degree from the rest. This causes the light to spread out, forming the dispersion spectrum.
It is now known that light behaves in many ways like a wave, with many different wavelengths. Different wavelengths are bent to a greater or lesser degree according to the refractive index of the substance they pass through such as glass or water. Longer wavelengths are always bent more sharply than shorter wavelengths; this is why the color spectrum of white light is always arranged--in order of decreasing wavelengths--red, orange, yellow, green, blue, indigo, and violet. This order is often remembered using the mnemonic "ROY G BIV."
In addition to prisms, there are other methods for obtaining the spectrum of light. Early scientists would shine light through a narrow aperture; this would cause the light to bend outward slightly, producing a small spectrum. By constructing a wire mesh called a diffraction grating, a much wider and more sharply defined spectrum could be produced. More modern diffraction gratings are constructed by scoring the surface of optically flat glass.
As scientists studied the spectra of sunlight, candlelight, and starlight, they began to notice tiny dark lines crossing the colored bands. By calculating the positions of these lines, they were able to determine the elements that were contained in the light's source. The science of examining spectra and spectral lines is called spectroscopy. In addition to visible light, spectroscopy also examines the spectra of infrared radiation, ultraviolet radiation, and x-ray radiation.
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