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1 THE PRISM SPECTROMETER OBJECTIVES : 1) Learn the theory of the prism spectrometer, and be able to explain the functions of its various components. 2) Observe the spectrum of a mercury discharge lamp and record the angle of deviation for the spectral lines. 3) Determine the index of refraction of a glass prism for various wavelengths. 4) Use the calibrated prism to measure unknown wavelengths. 5) Observe color sensation caused by light of particular wavelengths. INTRODUCTION: When a beam of light is transmitted from air to glass, the ray is bent according to Snell's law sin θ air = n sin θ glass (1) where the angles are measured from the surface normal (the line perpendicular to the surface) and n is the index of refraction of the glass. The index of refraction is a dimension-less number and is a measure of how strongly the medium bends light. The greater n is, the more the light is bent. The index of refraction of air is 1. For glass, n varies from 1.3 to 1.8, depending on the type of glass and on the wavelength of the light. θ air θ glass air glass White light is made up of all the colors of the rainbow - red, yellow, green, blue, and violet. Different colors correspond to different wavelengths. Human eyes are sensitive to light with wavelengths in the range 390 nm (violet) to 750 nm (red) (1 nm = nanometer = 10 - 9 m). 300nm 400 500 600 700 violet indigo blue green yellow red wavelength(nm) ultraviolet infrared Range of human vision
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