With this device, Wollaston saw that the colors were not spread out uniformly, but instead, some ranges of color were missing, appearing as dark bands in the solar spectrum. In 1802, however, William Wollaston built an improved spectrometer that included a lens to focus the Sun’s spectrum on a screen. If the spectrum of the white light from the Sun and stars were simply a continuous rainbow of colors, astronomers would have little interest in the detailed study of a star’s spectrum once they had learned its average surface temperature. When Newton described the laws of refraction and dispersion in optics, and observed the solar spectrum, all he could see was a continuous band of colors. ![]() Although it is hard to see in this printed version, in a well-dispersed spectrum, many subtle gradations in color are visible as your eye scans from one end (violet) to the other (red). Continuous Spectrum: When white light passes through a prism, it is dispersed and forms a continuous spectrum of all the colors. (In fact, a rainbow is formed by the dispersion of light though raindrops see Note: The Rainbow feature box.) Because this array of colors is a spectrum of light, the instrument used to disperse the light and form the spectrum is called a spectrometer.įigure 2. If the light leaving the prism is focused on a screen, the different wavelengths or colors that make up white light are lined up side by side just like a rainbow (Figure 2). Upon leaving the opposite face of the prism, the light is bent again and further dispersed. This phenomenon is called dispersion and explains Newton’s rainbow experiment. The violet light is bent more than the red. The bending of the beam depends on the wavelength of the light as well as the properties of the material, and as a result, different wavelengths (or colors of light) are bent by different amounts and therefore follow slightly different paths through the prism. Upon entering one face of the prism, the path of the light is refracted (bent), but not all of the colors are bent by the same amount. Action of a Prism: When we pass a beam of white sunlight through a prism, we see a rainbow-colored band of light that we call a continuous spectrum.įigure 1 shows how light is separated into different colors with a prism-a piece of glass in the shape of a triangle with refracting surfaces. Newton found that sunlight, which looks white to us, is actually made up of a mixture of all the colors of the rainbow (Figure 1).įigure 1. In 1672, in the first paper that he submitted to the Royal Society, Sir Isaac Newton described an experiment in which he permitted sunlight to pass through a small hole and then through a prism. For now, we turn to another behavior of light, one that is essential for the decoding of light. We will discuss astronomical instruments and their uses more fully in Astronomical Instruments. Small optical devices, such as eyeglasses or binoculars, generally use lenses, whereas large telescopes depend almost entirely on mirrors for their main optical elements. Such instruments are generally combinations of glass lenses, which bend light according to the principles of refraction, and curved mirrors, which depend on the properties of reflection. ![]() Reflection and refraction of light are the basic properties that make possible all optical instruments (devices that help us to see things better)-from eyeglasses to giant astronomical telescopes. Light is also bent, or refracted, when it passes from one kind of transparent material into another-say, from the air into a glass lens. If the surface is smooth and shiny, as with a mirror, the direction of the reflected light beam can be calculated accurately from knowledge of the shape of the reflecting surface. For example, light can be reflected from a surface. Light exhibits certain behaviors that are important to the design of telescopes and other instruments. Let’s examine how we can do this and what we can learn. ![]() To extract this information, however, astronomers must be able to study the amounts of energy we receive at different wavelengths of light in fine detail.
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