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Measuring wavelength with Young's slits - Teacher's Notes

Practical Advice

This experiment is worth spending time on. Taking the trouble to prepare the apparatus stacks the cards in favor of a successful set of fringes being produced. A reasonable blackout is still required. The assembled apparatus is unwieldy and thought needs to be given to arranging the laboratory to ensure smooth movement in semidarkness and to prevent the lamp of one student preventing another from seeing fringes.

You will need to issue instructions which enable the students to calculate the spacing of their slits from the number of turns on the thread. This will depend on the construction of your ruling device. In any case allowing them to calculate the distance moved for 10 turns is good practice.

A common failure is to rule too heavily, thus coalescing the slits.

Practiced teachers can usually tell slits which are likely to work by holding them up to the light. Students usually achieve success within three attempts.

It may be worthwhile producing some composite filters, half red and half blue, so that the colour can be shifted rapidly and the fringe spacing noted. Another approach is to have a red filter above a blue filter, producing two sets of fringes on the screen at once. Whichever strategy is adopted the aim is to make plain the increase in fringe spacing, as the wavelength increases.

Technician's note: Three coated slides per working group is probably enough.

Alternative Approaches

Laser demonstrations of two slits can replace this activity. This is to lose the simplicity of the approach. Being able to measure the wavelength of light emitted from an incandescent filament lamp makes the whole more approachable and avoids the special status that the laser sometimes acquires. By all means use a laser later to reinforce, but after the students have made this measurement.

Social and Human Context

Thomas Young (1773–1829) was something of a polymath – a student of Syriac and Hebrew: he later contributed significantly to the decoding of the Rosetta stone, being something of an Egyptologist. Interestingly he proved a failure as a public lecturer, resigning as Professor of Natural Philosophy at the Royal Institution after only two years. As another snippet on an interesting man he was elected FRS for his work on accommodation and the ciliary muscles in the eye in 1794.

Level

Adaptable
 

Last modified: 28 June 2002