Now that you’ve learned a bit about what diffraction gratings do to monochromatic (single color) light, it’s time to use them to look at more complicated light sources.

In this lab, you will use a device called a spectrometer to perform precision measurements of the wavelengths of light emitted by or absorbed by different materials. These collections of light are called spectra, and every material has a unique spectrum, kind of like an optical fingerprint. By measuring the spectrum of an unknown material, you can identify what it is made of.

By the end of this lab, you will have…

• … learned to use an optical spectrometer;
• … observed the emission spectrum from an incandescent bulb;
• … observed emission spectra from different isolated gases and identified the emission wavelengths; and
• … observed the absorption spectrum of holmium perchlorate and identified the absorption wavelengths.

The TA will introduce you to the spectrometer and how to use it. You will warm-up and practice using it by looking at the spectrum of an incandescent bulb and then measuring the emission spectrum of hydrogen. When you’re ready, you can then use your spectrometer to look at additional emission spectra from different gases.

Remember to write down everything you see and do in your group lab notebook.

• Make sketches, do calculations, take notes, and record observations. Your TA may ask to see your notebook during lab, and will read over it afterwards to assign a grade to the group.
• Use the digital group lab report to communicate the process and results from your experiment.

In the previous lab, you tested the relationship

where $\lambda$ is the wavelength of light, $D$ is the spacing between slits on your diffraction grating, $M = 0, 1, 2,...$ is the order of diffraction, and $\theta$ is the diffraction angle.

You also determined that the spacing between the slits on the glass diffraction grating was $D = 1.67 ~\mu\textrm{m} = 1.67 \times 10^{-6} \textrm{ m}$.

These facts will be useful to you in today's lab!

In this lab, you will use the grating spectrometer illustrated in Fig. 1.

Figure 1: Grating spectrometer

You will need to make a few preliminary adjustments before accurate wavelength measurements will be possible. For the sake of brevity, we have done some of the alignment procedure for you in advance. Time will not allow the most precise measurements possible with this apparatus!

1. Turn the collimator focus knob until the pencil line on the collimator tube just shows. (See Fig. 2 below.)
2. Place the light source at the collimator slit and turn the telescope to look back toward the collimator. While looking through the eyepiece, turn the focusing knob on the telescope (not the collimator) until the collimator slit edges appear sharp. You should see a set of crosshairs. Rotate the eyepiece until the crosshairs are vertical.
3. Adjust the collimator slit to a moderately narrow line. Note that one edge of the slit remains stationary while adjusting the slit width. You will use that stationary edge as the reference point for the crosshairs.
4. Rotate the grating table until the grating appears normal (at right angles to) the collimator. Leave the grating table locked in this position.
5. While looking through the telescope, find the collimator slit and place the intersection point of the crosshairs on the stationary edge of the slit. Read and record the angular position in the Vernier scale window. This position represents the zero-order diffraction. All diffraction angle measurements are to be made relative to this position. The angle and Vernier scales are shown below in Fig. 3.

Figure 2: Collimator pencil line

In order to read the angular position, note that the main scale is in units of “degrees”, and the smaller, inner scale (the vernier scale) is in units of “minutes”. There are 60 minutes in a degree, so 15 minutes equals 0.25 degrees, 30 minutes equals 0.50 degrees, etc.

Following the example shown in Fig. 3, you can read angle from these scales as follows:

• Read down from the zero mark on the vernier scale to the next smaller line on the degree scale. (In the example of Fig. 3, the result would be 20.5 degrees or 20 degrees and 30 minutes.)
• Next find the line on the vernier scale which best coincides with a line on the degree scale. (In the example, the best alignment is at 15 minutes.)
• Finally, add the two readings. (In the example, 20 degrees 30 minutes + 15 minutes = 20 degrees 45 minutes, or 20.75 degrees.)

Figure 3: Angle scale reading 20 degrees, 45 minutes

Consider: How do you measure angle? What do you see from the incandescent lamp? What range of wavelengths does visible light cover? Do you see first and second order diffraction? Do you see the spectrum on both sides of zero?

Consider: How does the hydrogen spectrum differ from the incandescent bulb? Can you identify all the hydrogen emission lines?

Consider: Can you match the lines to emissions from gases available in the lab?

Consider: How do you need to arrange your light source and sample to measure an absorption spectrum? Do you see the faint absorption lines? Do your measured values match expectations?

At the end of the lab, you will need to record your final conclusions (about 1 or 2 paragraphs) in your lab report summing up the important results and take-away points from your experiment. Remember that you should only draw conclusions which are supported by the data, so be ready to back up any statements you make!

When you're finished, save your file as a PDF and upload it to Canvas. (Only one student needs to submit the report, but make sure everyone's name is on it!) If you make a mistake, you can re-submit, but work done after the end of the lab period will not be accepted.