Teacher and Technician InformationYoung's slits

What it is for

This variation on a standard experiment is designed to enable students to see clear two-slit interference fringes, even in laboratories where a good blackout is not obtainable. The extra effort in setting up this apparatus is repaid by the quality of the fringes observed.

How to make it

Start with 2.5" or 61 mm square down pipe, available from builders merchants in 4 and 5.5 m lengths in brown, black or white. Black works well. You will also need some down-pipe connectors, this being the neatest way to support the tubes at the middle and screen end, where insertions need to be made into the tube. As a guide to costs; 4 m was quoted at £13.12 plus VAT; 5.5 m at £18.04 plus VAT. A supply of softwood for the supports, cardboard for the external baffle, hardboard for the internal baffle, is needed.

Be careful to choose the height of the supports such that the 36 W lamp is aligned with the center of the tube, when placed in a standard lampholder. If ground glass screens are not easily available, then taunt greaseproof paper makes a useful alternative.

The internal baffle is essential to prevent unwanted reflections within the tube.

Students will be making the slits to insert in the center of the tube whilst doing the experiment, although you may like to keep one set that work well, to show the whole set up.

Important features

… Note that the tubes are unequal in length. The shorter tube is nearest the lamp. The difference in length is important.

… The internal baffle must be in place to prevent stray reflections within the tube.

… The whole needs to sit securely on benches, with one end adjacent to an electrical supply, and still leave space to move around the laboratory.