From Flame Tests to Lasers: Extending Inquiry in Atomic Structure

Every year I begin chemistry with the flame test lab. It is a simple and colorful way for students to wonder about electrons. They see different salts burn with different colors, and they start asking questions about why that happens. Those questions lead into the Bohr model, ground and excited states, and eventually orbital diagrams. In my own framing of the inquiry cycle, this lab is the “call to adventure.” Students engage with something mysterious and exciting. The next steps of exploration and explanation follow naturally as they try to connect color to electron energy levels.

Where I have often struggled is the extend phase and first year chemistry students can find quantitative line spectra, a classic extension, overwhelming at this point in the course. What I noticed, however, is that many of them become curious about visible light itself. They want to understand wavelength, color, and how light relates to the fall of electrons. This year I leaned into that curiosity by exploring lasers as an extension of the flame test. At the atomic level, lasers and flame tests share the same foundation: electrons are excited, fall back, and emit photons of very specific energy. The difference is that lasers add amplification through mirrors, which produces an intense beam of a single wavelength.

To make this connection hands-on, we used inexpensive red, green, and purple laser pointers along with a simple diffraction grating made from the back of an old CD. Shining each laser on the CD interferes with the light waves in a specific way that creates bright spots at intervals related to the wavelength of the light.  The spacing of those spots is related to the wavelength of the light. Students measured the distance between the bright spots and the central beam for each laser, then compared the ratios. By setting up proportions, they could use the known wavelength of one laser, such as red at about 650 nm, to estimate the wavelength of green or purple. This was surprisingly accurate and gave them a sense of how scientists measure what we cannot directly see. It became a natural extension of the flame test: the unseen behavior of electrons revealed through the visible evidence of light.

Steps for the Laser Diffraction Lab

1. Gather materials: red, green, and purple laser pointers, a CD or DVD (shiny side out), a meter stick or ruler, a piece of graph paper taped to the wall.
2. Set up the diffraction grating: tape the CD upright so that the laser beam shines directly on its surface. Mark the central bright spot on the wall.
3. Measure diffraction: for each color laser, measure the distance from the central spot to the first diffraction spot, as well as the distance from the CD to the wall.
4. Calculate proportions: use the red laser as a reference. Create a ratio of diffraction spacing (red to green, red to purple) and compare it to the known wavelength of red light. Solve for the approximate wavelengths of the other colors.
5. Discuss energy: connect the results back to the flame test. Shorter wavelengths correspond to higher energy transitions, showing again how the color of light is linked to the behavior of electrons.

The below video outlines the general procedure for creating the diffraciton grating discussed above: