Solar Absorption Spectrum I: Diffraction (PHYS 110)

Most sources of energy on Earth receive their energy from the Sun. The Sun, on the other hand, produces energy through the process of nuclear fusion whereby hydrogen atoms are “fused” to form heavier elements. But how do we know that the Sun produces energy in this manner? How do we know that hydrogen even exists in the Sun?

Our knowledge of the Sun is limited to what we can deduce from afar. We note, for example, the gravitational effects the Sun has on other objects and we can see specific features with telescopes and satellites. But perhaps our greatest source of information is the light (and other radiation) that the Sun emits.

In this two-part experiment, you will use a simple instrument called a spectrometer to carefully examine the spectrum of visible light from the Sun. Within this spectrum, you can identify dark bands – called Fraunhofer Lines – and use these lines to identify potential elements in the Sun’s atmosphere. This technique – absorption spectroscopy – can be used not only to identify elements in the atmosphere of the Sun, but in the atmospheres of planets and moons anywhere in the solar system.

Today’s lab will prepare you to make these measurements by first introducing the diffraction grating and looking at the effect such an object has on light. By the end, you will have more familiarity with the component parts of a spectrometer, and will be ready to use a more precise version to make your measurements of the solar absorption spectrum when you return for the second part.


By the end of this lab, you will have…

  • … observed the behavior of light passing through a diffraction grating and determined which parameters affect the diffraction pattern;
  • … characterized a diffraction grating by determining the grating separation, $d$, and quantified your confidence in this value; and
  • … determined how a diffraction grating can be used to construct a spectrometer, which can be used to measure an unknown wavelength of light.


You do not need to know the theory behind a phenomenon in order to conduct an experiment. In fact, many – if not most – new phenomena are discovered by experiment before they can be explained by theory.

In today’s lab, you will play with diffraction gratings to observe how light behaves when it passes through them. To start off, you have been given two gratings and three different lasers. Your initial goal is to determine the following:

  • Which phenomena are common to all three lasers and which depend on the color of laser?
  • Which phenomena depend on the particular grating and which seem universal?

You should take notes as you go so that you can make some concrete statements about what you’ve observed. Write down everything you see and do in your lab notebook, including pictures, comments, data, calculations and conclusions. You will be asked to share your findings with the class partway through the lab.

WARNING: In this lab, you will be working with lasers operating at three different wavelengths – one each in the red, green and blue-violet portions of the spectrum. These lasers are low power and safe to handle without special eye protective goggles. However, you should always take care when handling lasers. Never look into the beam, nor direct it at another person.

At the midpoint, your TA will share an equation relating some features of the grating, of the pattern made by the grating, and of the laser. Your task is to use the known/measured quantities to determine the unknown quantities. In addition, you should be able to characterize how confident you are in your findings.

Group lab notebook

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.
  • All members are expected to contribute, read over, and comment on the report and all members of the group will receive the same report grade.

Outline of the lab

Make preliminary observations

Consider: What does the grating do when you shine light through it? Do the different colors of light behave the same way or differently? Do different gratings behave the same way or differently?

Make measurements

Consider: What parameters can you measure with your apparatus? What variables change as you swap out grating or laser pointer? What variables stay the same? Can you make quantitative measurements with uncertainties?

Determining groove spacing

Consider: How can you use what you’ve observed so far to determine the groove spacing? How do different gratings with different spacings compare?

Building a spectrometer

Consider: How would you construct a spectrometer to observe the solar spectrum?


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.

Remember to log out of all your accounts after you submit!