Absorption Spectra



A slide projector is used as a light source. A free standing prism is placed directly in front of the projector and a cuvette slide is placed in the slide holder so that a full spectrum appears on the screen. When glass blower's goggles are placed in the beam of light before it hits the prism, a fairly discrete band in the yellow range is darkened. When diluted food color is placed in the cuvette slide a larger band of the spectrum is darkened. 


Curriculum Notes 

Allow about 12 minutes for this demo.

One day of lead time is required for this project.



A projected spectrum demonstrates that specific frequencies of light are absorbed as electrons are excited to higher energy levels. Photons with specific energies (frequencies) are capable of exciting electrons in atomic or molecular orbitals into higher energy levels. The photons are absorbed in the process, therefore the portions of the spectrum corresponding to those frequencies are attenuated (darkened).



  • slide projector and small piece of wood to elevate the front end
  • diffraction grating and stand
  • diluted food color in dropper bottle
  • cuvette slides
  • glass blower's glasses with didymium glass lens


The projector will be set up facing the screen with the diffraction grating directly in front of the lens. Place an empty cuvette slide in the slide holder with the foil side facing the lamp. Turn on the projector. Dim the house lights. Focus the image by adjusting the lens on the front of the projector. A full spectrum will appear. Hold a lens of the neodymium glassblower's goggles directly in front of (the side closer to the screen) the diffraction grating. Fairly discrete dark bands will appear in the spectrum where the neodymium atoms' electrons absorbed energy. Set aside the goggles and turn off the projector.

Using the dropper from the dropper bottle, put enough diluted food coloring into the cuvette slide to the cover the slit. Place the cuvette slide into the slide holder with the foil side facing the lamp. Turn on the projector. A broad band of absorption will appear in the lower half of the spectrum. The upper half will be a full spectrum for comparison. Turn off the projector.


Safety Precautions 

Be sure that the foil side of the cuvette slide is facing the projector lamp. Otherwise the plastic slide could melt from the heat of the lamp.