Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Interference of water waves from two gaps

Demonstration

Plane waves in a ripple tank strike two narrow gaps. Each gap produces circular waves beyond the barriers, and the result is an interference pattern.

Apparatus and materials

Dippers, 2

Hand stroboscope

Motor mounted on beam, with beam support

Side barriers (blocks of wood)

Barrier, short

Ripple tank and accessories

Health & Safety and Technical notes


Beware of water on the laboratory floor. Make sure you have a sponge and bucket handy to mop up spills immediately. 
 
Place the power supply for the lamp on a bench, not on the floor by the tank. 
 
Photo-induced epilepsy 
In all work with flashing lights, teachers must be aware of any student suffering from photo-induced epilepsy. This condition is very rare. However, make sensitive inquiry of any known epileptic to see whether an attack has ever been associated with flashing lights. If so the student could be invited to leave the lab or shield his/her eyes as deemed advisable. It is impracticable to avoid the hazardous frequency range (7 to 15 Hz) in these experiments.

The side barriers prevent stray edge effects

 

Procedure


Ripple tanka Set up two straight barriers with a short one between them, along a line parallel to the beam and 10 cm to 15 cm away from it, as shown in the diagram. Make the gaps between barriers about 1 cm wide. 

b Generate straight waves with a wavelength of about 1 cm. 
 
c Draw students’ attention to the pattern made after the waves emerge from the gaps. If they need to freeze the pattern, offer hand stroboscopes. 


Teaching notes


A straight, parallel wave striking the two narrow gaps changes into two circular waves and interference effects are seen beyond the barrier. The pattern is the same as would be produced by two dippers vibrating in phase at the gaps, but fainter. The waves emanating from the two gaps carry less energy than waves produced by two dippers. Once students accept that each gap produces waves like those of a feebly moving dipper, they know what pattern to look for.  

This experiment demonstrates the technique that Young used to show the interference of light. By dividing a single wavefront, he hit upon a way of generating two point sources of waves that remain in phase with each other. 
 
Note: The nodal lines will spread out along hyperbolic paths and so the simple Young's fringes formula will not apply. 
 
fringe separation = wavelength x distance from vibrator / slit separation 
 
This template with two sets of semi-circular lines can be used with an OHT to simulate the interference of coherent waves from two point sources. 
 
This experiment was safety-checked in January 2007