Below are some examples of diffraction as seen through a diffraction grating similar to the one you will use in this lab. These images nicely illustrate the beauty and art inherent in physics. We should never allow ourselves to become so immersed in the mathematical mechanics of solving physics problems that we fail to see the beauty in how it manifests itself in the world around us.

The above images were taken in one of the teaching labs in KPTC by simply holding an inexpensive diffraction in front of a cell phone camera. I was working on developing material for this lab and the diffraction patterns I was seeing from the light sources that you will be working with inspired me to look for diffraction in other parts of the world around me.

In this lab you will study the properties of a diffraction grating like the one used to create the images above. You will then use that grating to study some atomic emission spectra in a manner similar to the pioneering research done in the 1800's which let to the birth of atomic physics and eventually the development of quantum mechanics.

One member of the group should click on the link below to start your group lab notebook. (You may be asked to log into your UChicago Google account if you are not already logged in.) Make sure to share the document with everyone in the group (click the “Share” button in the top right corner of the screen) so each member has access to the notebook after you leave lab. Choose one member of your group to be the designated record-keeper.

If you need a refresher on Diffraction and how a diffraction grating works see this page.

A diffraction grating uses diffraction from a large number of very closely spaces rulings (think of a ruling as a slit) to spread light out according to wavelength. The development of diffraction gratings allowed scientists to conduct experiments which revealed that when sufficiently heated, each element in the periodic table emits a unique spectrum of light containing specific wavelengths.

You have encountered the theory behind how diffraction gratings work in your lecture. Here you will make use of them to study some atomic spectra. Before moving on to looking at spectra, you will need to determine the number of rulings per mm of the grating used in your spectrometer.

Below is a photo of one possible setup for determining the number of rulings per mm of your grating.

You don't need step by step instructions for how to make this measurement, at this point you are perfectly capable of doing it on your own. However here are a few things you should pay particular attention to.

• The laser beam needs to be orthogonal to the plane of the grating. Why? Go ahead and observe what happens when the beam is not orthogonal to the grating. How will you make sure that this condition is met?
• How will you most accurately determine the angle of the diffracted beams?
• How will you estimate the uncertainty in the measurements?

You will now proceed to use a diffraction grating spectrometer to study the emission spectra of Hydrogen and other gaseous elements. You may not yet have encountered the modern physics theory which explains the energy level structure of the elements which gives rise to their emission spectra. You will encounter this material in PHYS234 and PHYS235. Don't let the fact that you are doing an experiment on a subject whose theory you have not yet encountered. It was early experimental investigations of atomic energy spectra using diffraction grating spectrometers which spurred the development of modern atomic theory, initially with the Bohr model of the atom and then ultimately giving rise to the development of quantum mechanics.

The optical spectrometer you are using is a precision instrument. If however you have not spent much time looking through the telescope/microscope eyepieces, you may find it difficult to see the spectral lines. The following section walks you through the process of setting up and using the optical spectrometer with an Hg lamp as a light source.

Figure 9: Grating spectrometer

You will use the grating spectrometer illustrated in Fig. 9. A few preliminary adjustments will be needed before accurate wavelength measurements will be possible. For the sake of brevity, we have done some of the alignment procedure for you in advance.

1. Turn the collimator focus knob until the pencil line on the collimator tube just shows.
2. While looking through the eyepiece, slide the eyepiece in and out until the crosshair appears sharp.
3. Place the light source at the collimator slit and turn the telescope to look back toward the collimator. Turn the focusing knob on the telescope (not the collimator) until the collimator slit edges appear sharp.
4. Adjust the collimator slit to a moderately narrow line.
5. Rotate the grating table until the grating appears normal (at right angles to) the collimator. Leave the grating table locked in this position.
6. 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.

Figure 10: Angle scale reading 20 degrees, 45 minutes

In order to read the angular position, proceed as follows (consulting Fig. 10 as an example):

• Read down from the zero mark on the vernier scale to the next smaller line on the degree scale. (In the example of Fig. 10, 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.)

To get used to using the spectrometer find and measure the wavelength of as many of the emission lines of Hg as you can find. For reference the most prominent lines in the Hg spectrum are tabulated below.

The primary goal of this previous part of the lab was to learn how to make accurate measurements using the spectrometer. Now you are tasked with characterizing the spectrum of the Hydrogen atom. Replace the Hg light source with the Hydrogen discharge tube. Note that your tube may be labeled Deuterium, which is just hydrogen with a neutron added to the nucleus and for out purposes has the same spectrum as Hydrogen). You may have to re-measure your 0th order diffraction angle on the spectrometer.

The lines which you are seeing are part of what is called the Balmer series in the spectrum of Hydrogen. In PHYS234 you will learn more about the energy level structure of the Hydrogen atom and thus where the Balmer series of lines comes from. For now, keep in mind that one of the fundamental roles of experimental physics is the discovery of new phenomena. The fact that you are observing and quantifying something in the lab without yet knowing the underlying theory is mimicking how research experiments lead to the development of new theoretical models. Never forget that experiments are not done to verify theory. Theory follows experiment, not the other way around.

There are a number of other discharge tubes containing other elements than Hydrogen.