A. Purpose: To determine the Index of Refraction of the prism material as a function of wavelength, to find out an unknown element from the calibration curve of Helium spectrum and, to find the variation of the speed of light with wavelength in the prism material.
B. Equipment: Spectrometer, prism, prism clamp, spectral lamps, lamp housing, goose lamp, spirit level and card boards.
CAUTION: Lamp at high voltage. Do NOT touch it while in operation. Handle the glass prism and the lamps with care. Turn off the lamp immediately after the experiment is over. Unplug the electrical outlet. Do not remove the eyepiece from the telescope or touch the eyepiece lens.
C. Introduction: Present knowledge of atomic physics grew out of simple spectroscopic experiments such as the one you will perform in this lab. Light has been a fascinating subject of studies for many years and only partially understood even at present. Light is also one of our most powerful tools for probing nature. The atoms of a substance when sufficiently disturbed, emit a unique set of wavelengths of light. The ordinary light as seen by the naked eye, that is actually a broad continuous band of colors (wavelengths) when viewed with a prism spectrometer. A ray of light on passing from one medium to another changes direction in accordance with one of Snell's Laws, known as law of REFRACTION. The angular deviation depends on the medium under consideration and also on the wavelength of light. The Prism Spectrometer makes use of refraction as well as an effect called DISPERSION. The speed of light in a medium depends on wavelength. The change in the index of refraction with wavelength, is called dispersion.
D. Outline of the Experiment: Incident light from the lamp is passed through a narrow opening of the collimator to produce a thin ray of light. This ray from the collimator passes through a prism placed at the center of the circular table and reaches the telescope for viewing. The composite light from the collimator suffers dispersion in the prism and spreads out into component colors at different angles of deviation. By measuring the angle of deviation, one can determine the index of refraction of the material from given formula. Calibration curve of the deviation angle versus wavelength of the Helium spectrum is used to determine the unknown element.
1. Preparation of the Experimental Arrangement: Use spirit level to make the prism table completely horizontal. Turn the He lamp on and place it near the entrance slit of the collimator. Look through the telescope directly towards the slit and adjust the eyepiece to make the cross hairs and the image of the slit distinctly sharp and bright. The slit of the collimator should be narrow. You may have to adjust the position and distance of the lamp from the slit for maximum brightness of the image. Measure the initial direction of the light by recording the position of the telescope. Place the prism at the center of the prism table and clamp it. Adjust the height of the prism table. Use the ring underneath to secure the height. Look through the telescope and move it around to see the color images of the slit. You should be able to count at least nine color images of the slit. These are the spectral lines of Helium. Rotate the prism table while keeping the spectrum in view until it changes direction. Stop rotating the prism at this position of the prism. This is called the position of minimum deviation. Seek help if you do not know how to fix the position of minimum deviation. The lamp position, the slit width and the prism position MUST NOT ever change during the course of the experiment. Find angle of the direct beam before and after the experiment.
2. The Experiment: Record the angular position of the telescope for all (about 9) lines of the He spectrum, each line at its position of minimum deviation. These angles are the final directions of the light from the prism. Prepare a table of data of these final angles together with color, brightness and wavelength. Use a scale of 5 (weak)to 10 (strong) for the relative brightness of each color. Make NO changes in the experimental arrangement except replacing the He tube with a tube of unknown element from your lab instructor. Repeat measuring the final directions of the spectral lines of the unknown lamp at minimum deviation. The angle of Minimum Deviation D is obtained from the difference between the initial angle of incidence and the final angle out of the prism. Record your data of the unknown tube in the same table for He spectrum. Observe the spectrum of a tungsten lamp. Is it different? Explain. What is the origin of the discrete or continuous spectrum?
Plot a graph of Minimum Deviation angle versus Wavelength for the He spectrum. Draw a continuous smooth curve as a best fit of data for the Helium spectrum. Use this curve called Calibration Curve for determination of the wavelengths of the unknown lamp and to identify the unknown element from table* below. Calculate the Index of refraction of the prism material and the speed of light in the prism for the nine lines of the He and, for the unknown element. Plot a graph of speed of light versus wavelength. Keep at least 4 significant digits in your calculations. Calculate the speed of light for each index of refraction. How does the speed of light change from red to blue? Will the speed of different colors be different in vacuum?
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* All wavelengths are in angstrom, 1 Angstrom = 1X10-10 m
Use EXCEL software for calculation. Index of Refraction is given by,