Interference with plastic wave model

Class Practical

Using strips of plastic to model what happens to at different positions in terms of changing phase relationships.

Apparatus and Materials

• Plastic waves, 2
• Nails, 15-cm, 2
• Retort stand
• Bosses, 2

Health & Safety and Technical Notes

Read our standard health & safety guidance

Procedure

1. You can illustrate interference from two point sources by crossing the ends of the plastic waves supported from a retort stand, as shown.
2. Move the crossover point up and down to show that light and dark bands will be produced on a screen.

Teaching Notes

• This apparatus can be used to explain Young's experiment, with light producing fringes.
• With more able students, you could increase the spacing of the clamps to get more lines of nodes. It is not advisable to start with wide spacing as the waves then cross at quite large angles in some cases and the addition and subtraction are not so obvious.
• If you prefer to work in the horizontal plane, you could place the two vertical rods about 25 cm apart to represent the slits. Support the plastic waves from them in a sling so that they can be moved horizontally backwards and forwards.
• An alternative to plastic waves is corrugated cardboard. Cut two strips about 20 cm long and about 1/4 cm wide, and place them on their sides on a drawing board. Pin each strip to the board by a pin through a wave hump near one end. These anchored ends represent the two sources, a few centimetres apart on the board.

This experiment was safety-checked in February 2006