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Spectra



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SPECTRA White light is a mixture of all wavelengths When white light is sent through a prism or a diffraction grating it is broken up into a continuous distribution of colors called a spectrum The relation between wavelength and color is given in the following table color wavelength nm violet 400 450 blue 450 480 blue green 480 510 green 510 550 yellow green 550 570 yellow 570 590 orange 590 630 red above 630 A rainbow is a spectrum produced when sunlight is refracted through raindrops Any thin slice of color in a continuous spectrum is called a spectrally pure color and is characterized by its wavelength Spectrally pure colors are rare in most of our natural environment but they are easily produced in the laboratory When an electric current is sent through a diffuse transparent gas the gas is heated and glows If the light from such a gas is passed through a prism or diffraction grating a discrete set of bright colors will typically be seen When a diffraction grating is used the colors appear to be narrow lines parallel to the slits of the grating and are called spectral lines A collection of these lines usually in the form of a table or graph for a gas composed of a given chemical element is called the line spectrum of the element A very different sort of spectrum is produced by a hot opaque object like a lightbulb filament Instead of discrete spectral lines this incandescent light source produces a broad distribution of colors Every color in the visible range is represented in such a spectrum and the relative intensities of various colors depends on the temperature of the hot object 3 1 Experiment 1 Grating Spectroscope Apparatus CENCO grating spectroscope variac controlled light bulb sodium and mercury vapor lamps Procedure Hold the device so that its right angle is on your left View through the grating fixed to the aperture at the short end Illuminate the vertical slit on the left with the light to be studied The nominal wavelength is read on the scale to the



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