The grating supplied will be mounted in the special grating holder for the spectrometer. The gratings are made from a master grating which has 300 grooves per millimeter (or 300 lines/mm). However, due to contraction in the transfer process, the actual number of lines/mm for your grating will most likely be slightly different from this nominal value. For accurate work the grating must first be calibrated using an emission line of known wavelength. For calibration we will use the so-called D-lines of sodium. These are two closely spaced lines (so close our instrument cannot resolve them) and the weighted average wavelength is 589.21 nm.

1.  The sodium lamp requires some time to warm up, and it should be turned on at the start of your work and kept on until you obtain all of the data.

2.  Examine the spectrometer, which is schematically illustrated in Fig. 16-1 above, and acquaint yourself with the components. DO NOT CHANGE the settings of the three screws on the underside of the prism table, nor the screws that change the tilt of the collimator or of the telescope. The collimator consists of an entrance slit and a lens mounted at a distance of f from the slit, where f is the focal length of the lens. Therefore light coming from the slit and through the lens will have parallel rays. No adjustment should be necessary on the collimator. The spectrometer mounting table for the grating at the center of the spectrometer is called the prism table even though we will be using a grating in this experiment because the instrument can also accept a prism as the dispersive element.

The telescope holder can be rotated about the central axis by loosening the coarse rotation screw C and turning the fine rotation screw S. Handle only the arm to which the telescope is attached, not the telescope itself.

To adjust the cross hairs of the telescope eyepiece, place a piece of white paper in front of the objective; and, while looking through the telescope, slide the eyepiece in or out a short distance to bring the cross hairs into sharp focus. Next relax the eye by closing it for a short time or by looking toward the floor for a few seconds. Now look into the eyepiece and see if immediately upon looking, the cross hairs are as sharply defined as before. (The eye will accommodate for a poorly adjusted eyepiece but it will result in errors of parallax and also eyestrain). After adjusting the eyepiece, it should not be moved in the telescope tube for readings by the student who is looking through it. However, the adjustment will need to be repeated for a different eye.

Study the vernier of the spectroscope until you understand its operation. A small magnifying glass should be part of the equipment to enable you to read the vernier precisely.

3.  Put the sodium lamp source a few centimeters from the collimator slit. Aim the telescope directly on line with the collimator and adjust it by means of focusing thumbscrew to obtain a sharp image of the slit on the cross hairs. Using the magnifying glass, read the position of the vernier to the nearest minute. This is your "straight through" position. Record this as the zero reading.

4.  Tighten the telescope coarse rotation lock-screw C and use the fine rotation knob S to align the vertical line of the graticule with the fixed edge of the slit.

5.  Use the slit thumbscrew to adjust the slit width for a clear, bright image. Measurements of the diffraction angle are always made with the graticule line aligned along the fixed edge of the slit. Because of this, a very narrow slit is not necessarily advantageous.

6.  When the telescope and collimator are properly aligned and focused, the slit should be sharply focused in the center of the field of view of the telescope, and one cross hair of the graticule should be aligned with the fixed edge of the slit.

7.  Attach the grating mount to the spectrometer table using the thumbscrews. Be sure it is perpendicular to the engraved lines.

8.  Place the diffraction grating into the clips of the mount. Check the orientation of the grating. Look though the grating at a light source. If aligned properly, the light will be dispersed horizontally in various colors.

9.  Open the Worksheet and fill in the header information.

10.  Measure the diffraction angle (both right and left) for as many orders of the Na yellow line as you can. Record these data in the Table in cells A13:C16.

11.  Measure the angular positions of each of the visible lines in the hydrogen spectrum. Although calculations will predict four lines in the visible part of the spectrum, you will most likely see only three because the fourth is weak and difficult to detect. Measure the positions on both the left and the right for each line and for two orders. Record the angles in the Table in cells C27:D29 and C33:D35. As a guideline, the accepted values for the hydrogen lines are:

Accepted Wavelength (nm)		Apparent Color
656.2		red
486.1		blue
434.0		violet

♦♦♦ OPTIONAL---IF TIME PERMITS---CONSULT INSTRUCTOR♦♦♦

12.  Measure the helium spectrum. Record the right- and left-side angles in the Table in cells C42:D50 and C54:D62 for two orders. Again, for guidelines only, the accepted values are

Accepted Wavelength (nm)		Apparent Color
706.5		red
667.8		red
587.6		yellow
504.7		green
501.5		green
492.2		blue-green
471.3		blue
447.1		blue
438.8		violet