Practical Activity for 11-14
Students use diffraction gratings to observe the spectra from different sources and deduce how we can work out which chemicals are present in an exoplanet’s atmosphere.
Preperation and safety
Refer to CLEAPSS Laboratory Handbook 9.10.2 for Bunsen burner precautions. Warn students not to stare directly into the lamp.
Each group of students will need:
- Access to a filmant lamp and sodium lamp
- Access to other light sources (eg a fluorescent lamp, LED torch, gas discharge tubes)
- Bunsen flame
- Diffraction grating or spectroscope
- Sodium chloride – a few grains
- Marker Pen
Ask students to:
- Look at a filament lamp through the diffraction grating or spectroscope.
- Repeat with the other light sources.
- Look at the light coming from the Bunsen flame. Drop a few crystals of salt into the flame so that it turns orange.
- Watch you perform a demonstration of filament lamp shone onto a Bunsen flame and observe the shadow as sodium chloride is added to the flame.
- Watch you perform the same demonstration but with a sodium lamp in place of the filament lamp.
- Are all the colours of the spectrum present??
- Are any colours brighter than the rest?
- For the demonstration, what causes the shadow?
It is important that students can observe a number of light sources. You may wish to place several around the room and allow students to move around from one to another, recording their observations as they go. Alternatively, you could set up each source in turn at the front of the room so that all students can see the same source and spectrum at the same time.
Some students may find it difficult to observe a spectrum. If you have provided handheld spectroscopes show them how they can change the width of the slit to let more or less light in. If they are using unmounted diffraction gratings they should hold the diffraction grating close to one eye and look directly at the source. Then, by looking to one side, they should see a spectrum. It may help to use card or paper to cover most of the grating, leaving a small slit uncovered.
The demonstration introduces the idea of absorption of light. Students will be familiar with the idea of how shadows are formed, but they may not think of this as the absorption of light. They may never have thought about whether a gas can absorb light.
Explain how a spectrum can tell us about the elements present in an planet’s atmosphere.
Download the resources
The resource below includes teacher notes, a student worksheet and instructions.