DISPERSION OF A GLASS PRISM - Ryerson Department of Physics

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PH2 – page 6↑Closest to perfect alignmentFigure 7: Vernier reading, Example 2Main scale divides one complete revolution into 360 0 and the vernier has 10 divisions. Thevernier line closest to perfect alignment with the main scale line reads 0.8PROCEDURE: MEASURING THE ANGLE OF MINIMUM DEVIATION1. Place the light source a few centimeters behind the slit of the collimator.2. Once you have aligned and focused the spectrometer as described earlier, place the prism on thespectrometer table so that the incident light passes through it. It may be helpful to partiallydarken the room.3. It is generally possible to see the refracted light with the naked eye. Locate the general directionto which the light is refracted, and then align the telescope and spectrometer table base so the slitimage can be viewed through the telescope.4. While looking through the telescope, rotate the spectrometer table slightly back and forth. Noticethat the angle of refraction for the spectral line under observation changes. Rotate thespectrometer table until this angle is a minimum, and then rotate the telescope to align thevertical cross-hair with the fixed edge of the slit image as in Figure 8 below. Use the fine adjustknobs to make these adjustments as precisely as possible, then measure the telescope angle θ minusing the vernier scale.Slit ImageVertical Cross-HairFigure 8: View through the telescope
PH2 – page 75. Without changing the rotation of the spectrometer table, remove the prism and rotate thetelescope to align the cross-hair with the fixed edge of the unrefracted beam. Measure the angle θon the vernier scale. The difference between this angle and that recorded for the refractedspectral line in step 4, is the angle of minimum deviation. Notice that, since the determination ofthe angle of minimum deviation for each spectral line requires rotational adjustments of thespectrometer table, the angle of the undeflected beam must be remeasured for each line.6. The table below gives the wavelengths (in nm) of the spectrum lines that can be seen clearly inthe helium discharge. Record in the same column, the values of the θ 0 (the vernier angle with thetelescope and collimator axis aligned), θ min (the vernier reading at minimum deviation).Table of observations: Dispersion of Helium Light Through A Glass Prism.Colour ofthe Linesin theHeliumDischargeWavelength(nm)weak red 707strong red 668yellow 588green 502blue-green 492blue 471violet 447A ( o ) θ 0 θ minD min =θ min – θ 060 o7. Repeat steps 3 to 5 above for five other lines of the spectrum and record your observations in thetable above.DATA ANALYSIS1. For each of the analyzed lines calculate the angle of minimum deviation D min , and then the indexof refraction n, of the glass of the prism to four significant digits using the Eq. 1. Record yourresults in the table above2. Using the values in this table, plot a graph of the index of refraction vs. the wavelength of thesespectral lines. Analyze and comment on the shape of this graph.3. Could you use this graph to predict the index of refraction of the glass prism for a differentcolour, not present in the current spectrum?
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Page 5: PH2 - page 5READING THE VERNIER SCA

DISPERSION OF A GLASS PRISM - Ryerson Department of Physics

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