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 are provided with some inexpensive diffraction gratings, enough for each person in your group. Begin by simply using your eye-brain detector system to qualitatively study the behavior of light passing through the grating. Hold the grating up close to your eye and look at various light sources in the room such as a window, or the lights on the ceiling. At first what you see will be confusing and it can help to try and find a light source which is isolated from other sources of light to begin with. Try rotating the grating and observe what happens. Note that the diffracted light will appear off to one side of the source, so try looking searching the area around the source by moving just your eyes while holding the grating steady. After a bit of practice you should be able to observe a continuous rainbow like spectrum of light from different sources in the room. Some of these sources may have additional features within the continuous spectrum. Make note of your observations in your group notebook. In particular pay attention to the following:

• What happens to the diffracted spectrum of light when the grating is rotated?
• If the grating is held so that the diffracted light appears off to the left and right sides of the grating, what happens when you tilt the grating up/down or left/right?
• Do the lights in the ceiling produce the same spectrum of light as what you see out the windows? Note that there are two types of lights in the ceiling and the answer might be different for each type. What difference do you observe?

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. Not shown in this figure is the inclusion of a webcam which has been mounted to the eyepiece. This is a new addition to the lab which allows all partners to view the spectra on a computer screen. Previously only one person at a time was able to look through the eyepiece.

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. Connect the camera to a USB port on the computer and open the camera app.
   1. Switch the software from the default camera to the Logitech camera.
   2. The camera app should now show the view through the eyepiece on the screen.
3. Place the light source at the collimator slit and turn the telescope to look back toward the collimator.
   1. If necessary, rotate the eyepiece camera so that the illuminated slit appears vertical.
   2. 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.)

Now that you have spent some time qualitatively observing the spectra of different light sources it is time to make quantitative measurements. In order to measure wavelengths of the diffracted light, you will need to measure the groove spacing of the diffraction grating used in your spectrometer.

Using the optical rail and the lasers provided for you, develop a method of measuring the groove spacing of a diffraction grating. As a test case use one of the inexpensive gratings which has 1000 grooves per mm. Make sure that you can perform the measurements which accurately confirm the known groove spacing of these gratings before moving on to the grating used in the spectrometer.

At your station you have several different sources which emit light including: an incandescent bulb, a compact fluorescent (CF) bulb, a hydrogen discharge lamp (which may be labeled as deuterium which is simply an isotope of Hydrogen that has an emission spectrum so close to that of Hydrogen that they can be treated as identical for the purposes of this lab), and a mercury lamp. The incandescent and CF bulbs both give off a continuous spectrum of “white” light, but you should observe something else in the spectrum from the CF bulb.

In the discharge tube an electric current is used to excite hydrogen atoms which then emit light of very specific colors which we call an emission spectra, you will learn about the physics related to atomic energy levels and emission lines when you take quantum mechanics next year. The Hg lamp uses heat to excite Hg atoms creating an emission spectrum unique to Hg. As you will learn in quantum mechanics, each element has its own unique energy level structure which results in a unique emission spectra when those atoms are excited. The emission spectra of an element is like a finger print in that it uniquely identifies that element.

For fun there are emission tubes for several other elements such as He, and Ne in the lab. You can look at them just to see how each elements emission spectra is different, but you do not need to make any particular notes of these observations.

In the lab there is a poster showing a number of spectra, including spectra taken of sunlight here on Earth. Note the presence of vertical dark lines in the solar spectrum, these are called Fraunhofer lines in honor of the scientist who discovered them. These dark lines appear because light produced in the sun at these wavelengths was absorbed during its passage through the suns outer atmosphere. As you will learn from quantum mechanics, not only do elements emit light at specific wavelengths when excited, they also absorb light at those same wavelengths.

By measuring the wavelengths of the missing light in the solar spectrum, and comparing to the characteristic wavelengths of light emitted by different elements, it is possible to deduce which elements are present in the solar atmosphere.

The full solar spectrum shows many absorption lines, and can be confusing to analyze. The figure above shows the solar spectrum with only a subset of the lines to make it easier for you to compare with your measured emission spectra.

After the lab, you will need to write up your conclusions. This should be a separate document, and it should be done individually (though you may talk your group members or ask questions). Include any data tables, plots, etc. from the your lab notebook as necessary in order to show how your data support your conclusions.

The conclusion is your interpretation and discussion of your data. What do your data tell you? How do your data match the model (or models) you were comparing against, or to your expectations in general? (Sometimes this means using the $t^{\prime}$ test, but other times it means making qualitative comparisons.) Your conclusions should always be based on the results of your work in the lab. It is not acceptable to evaluate the results of an experiment by comparison to known values or any other form of preconceived expectation. Your conclusions need to be supported by your data. If your data are inconclusive or in disagreement with regard to your expectations then your conclusion should reflect that.