# Young's fringes with centimetre waves

##### Demonstration

Shows that an invisible part of the electromagnetic spectrum too produces interference fringes from two coherent sources.

#### Apparatus and materials

Microwave transmitter

Amplifier

Loudspeaker

Aluminium barriers

#### Health & Safety and Technical notes

Modern equipment using a solid-state diode transmitter is safe. Older equipment using a klystron tube uses hazardous voltages. The connectors on the leads between the transmitter and the power supply MUST be shielded types to minimize the risk of serious electric shock. The ventilation holes in the power supply may also give access to hazardous voltages, so its use MUST be closely supervised.

The microwave transmitter should be about 3 cm.

If the microwaves are unmodulated, simply connect the receiver to a meter. If they are modulated, it is possible to detect them using an amplifier and loudspeaker attached to the receiver.

Set up the transmitter symmetrically behind the two gaps, each of which is a half-wavelength wide.

#### Procedure

a Move the receiver in an arc, always the same distance from the gaps. The current through the meter should rise and fall to show about five maxima.

b Show that, when either gap is covered with a metal plate, the received signal decreases if the receiver is at a maximum but increases if the receiver is at a minimum.

Photograph courtesy of Mike Vetterlein

#### Teaching notes

If there is no risk of students thinking that the sound waves they hear are interfering with each other, it is worth modulating the transmitter and connecting the receiver to an amplifier and loudspeaker. The system then shows the change very clearly.

If you have a probe receiver (‘stick aerial’), use it rather than a horn receiver and you will get better results.

This might be an opportunity to discuss interference of radio and TV signals, when the direct signal interferes with its reflection from a building or an aircraft.

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