Atomic Spectra With a Diffraction Grating

Materials: ★★☆ Available in most school laboratories or specialist stores
Difficulty: ★★★ Requires a more experienced teacher
Safety: ★★★ Only to be attempted with adequate safety procedures and trained staff

Categories: Atoms, Electromagnetic Spectrum and Waves, Nuclear Physics

Alternative titles: Fingerprint of Atoms: Emission Lines, Emission Spectra

Summary

Using a diffraction grating and a gas discharge tube, you can observe the unique emission lines of different elements. These distinct line patterns reveal the quantized energy levels of electrons in atoms.

Procedure

Set up the spectroscope with the collimator and telescope aligned.
Place the gas discharge tube in front of the collimator and switch on the power supply.
Look through the telescope to view the central bright line of the emitted light.
Slowly rotate the telescope to one side to observe the first-order spectral lines.
Rotate further to find repeated patterns representing higher-order diffraction lines.
Record the colors and positions of the lines for comparison with known spectra.

Links

Seeing Atomic Spectra - SMUPhysics:

📄 How to See Atomic Spectra - Classroom Physics Demos: https://demos.smu.ca/demos/optics/67-spectral-lines

Variations

Try different gas tubes (hydrogen, helium, neon, mercury) to compare emission spectra.
Use a handheld diffraction grating to view spectra without a full spectroscope setup.
Measure the diffraction angles and calculate the wavelengths of the observed lines.

Safety Precautions

Handle gas discharge tubes carefully—they are fragile and can break if dropped.
Do not touch the powered electrodes of the lamp.
Ensure the lamp is turned off before changing tubes.

Questions to Consider

Why does each element produce a unique set of emission lines? (Because each atom has specific electron energy levels, and photons are emitted only when electrons drop between these levels.)
Why are the emission lines discrete rather than continuous? (Electrons can only occupy certain quantized energy states.)
How does a diffraction grating separate light into its components? (It diffracts and interferes with light waves so different wavelengths are directed at different angles.)
How could emission spectra be used in astronomy? (By analyzing light from stars, scientists can identify which elements are present in distant celestial objects.)