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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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Introductory Physics Experiments (Physics 252, v4.0) 2 Glass has a greater index of refraction at shorter wavelengths, that is, it bends blue light more than red light. So a prism can be used to disperse white light into its component colors. white red blue n λ wavelength blue red In this experiment, we will use a prism spectrometer to measure the dispersion angle of various wavelengths. From the measurements, we will make a graph of the index of refraction vs. wavelength. The form of the curve of index of refraction as a function of wavelength, known as the Cauchy formula, is n = A + B/ λ 2 Or n = A + (b/ λ ) 2 ***In the introduction for your lab , find B in terms of b, and solve the first equation for l , given n, A, and B*** As a light source, we will use a mercury lamp, which emits light at several discrete wavelengths. The device we are using is called a prism spectrometer because, once the prism is calibrated, it can be used to measure the wavelengths of the lines in the spectra produced by various atoms. The spectra contain bright lines at particular wavelengths, which correspond to light emitted during the transition between different energy states of the atoms. You see distinct lines because the atoms exist only in distinct, quantized energy states. Trying to explain the data from such experiments— the existence and pattern of sharp spectral lines—led to the development of quantum mechanics. When a ray of light is refracted by a prism, the angle between the incoming and
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This note was uploaded on 09/26/2011 for the course PHYSICS 106 taught by Professor Arubi during the Summer '11 term at UCLA.

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