Inexpensive diffraction grating glasses are used to observe the spectra of many light sources.
The diffraction gratings separate the colours contained in a light source. This can be used to illustrate everything from “bar codes” for chemical elements to colour temperature to lighting technology and so on. It is possible to do pretty good quantitative experiments with these glasses.
NCEA & Science Curriculum
Jnr Sci, PHYS 3.3
These glasses are very inexpensive and are very useful.
Qualitative for junior science
Use light sources in dark room. Students should wear the glasses. The tubes will need to be covered so that only a small slit of light is exposed. If possible face the light sources away from each other to prevent stray light affecting the observed spectra. The images above do not do justice to the visual observations. Increasing the distance between the observer and the light source will spread out the observed spectrum. Examples are displayed below.
Quantitative for physics 3.3
Although a qualitative demonstration with these glasses is enormously useful in itself, quantitative measurements can be made as well. For example, the bright sodium D line (589 nm) can be used to calculate the actual spacing of the glasses. Mark two points about 3 m apart and place the sodium discharge tube in the centre. Looking at the discharge tube through the glasses students walk away from the source until the perceived D lines line up with the two marks. The distance from the source is recorded. The separation of the source from one of the observed D lines can be used to calculate sin θ in the formula
where n = 1 for the first order maximum.
rearranged this gives
Results agree well with the 500 lines/mm given on the glasses.
With this number in hand you can measure the wavelength of other lines in other spectra.
These glasses are invaluable for investigating modern lighting technology. Have a look at a compact fluorescent bulb and an LED bulb. Both use fluorescence. With the compact fluorescent bulb you can see the visible part of the Hg bar code and the light produced by fluorescence, and the the LED you can see the driving light (far blue/violet), a fluorescence gap, and the fluorescence spectrum. Or put an old incandescent bulb on a dimmer to illustrate colour temperature and blackbody radiation. When the filament is warm blue and violet are pretty much missing. As you increase the temperature, making the filament hotter, all of the colours get brighter but the percentage of light in the blue and violet increases dramatically as expected for something approximating a blackbody. All very cool. Note: to improve results, you can put a cardboard slit in front of the light sources but be very careful not to let the cardboard get too close to the bulbs, particularly the incandescent as it will be very hot.
Avoid looking at the sun, lasers, or other bright sources.
Individual teachers are responsible for safety in their own classes. Even familiar demonstrations should be practised and safety-checked by individual teachers before they are used in a classroom.
Through the looking glass
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This teaching resource was developed in collaboration with Rory O’Keeffe, a New Zealand Science, Mathematics and Technology Teacher Fellow, 2007, hosted by Victoria University School of Chemical and Physical Sciences. Rory is a science teacher by training and was Deputy Principal at Lytton High School in Gisborne, New Zealand. He is also an amateur astronomer. See NZSMT Teacher Fellowships for more information about the Teacher Fellow Program.
This teaching resource was developed with support from