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

Hearing a laser beam

Demonstration

To show that a laser beam can easily be modulated to carry information.

Apparatus and materials

Laser (e.g. He Ne or diode type)

Strobe wheel

Photodiode light detector circuit

Audio frequency (AF) generator with amplification stage, or audio amplifier

Loudspeaker

Second version 

Small mirror mounted on any stiff surface e.g. a sheet of metal

Health & Safety and Technical notes


Check that the laser is labelled 'Class 2' and warn students not to stare into the beam.

Alignment of the beam on the photodiode is very important - if the beam is too strong the photodiode will saturate and not work well. You may need to adjust the beam so it falls just on the edge of the photodiode. You may also need to turn the lights off in the room - fluorescent tubes will cause the detector to hum at 100 Hz.
 
A photodiode detector circuit may be found in most electronics texts. A diagram of a very simple one is shown. It uses a 9 V battery, a 100 kWresistor and 0.1μF capacitor.

 

Procedure


Align the laser so that the beam falls on the photodiode. The photodiode can be connected to the amplification stage of the AF generator and the output of the generator goes to a speaker. Note the AF generator is used only as an amplifier - an ordinary audio amplifier would also do.
 
Place the hand strobe wheel so it intercepts the laser beam. Spinning the wheel interrupts the beam many times a second, causing the audio output to click or hum.
 
If the beam is first reflected off a small mirror mounted on a surface, then a click will be heard when the surface is tapped, e.g. with a pencil.

 

Teaching notes


The photodiode is crudely converting the presence or absence of light into a click on the speaker. Interrupting or modulating (i.e. changing the intensity) the laser beam, therefore, changes the resistance of the diode. This demonstrates how digital data can be carried by a laser beam.
 
The reflection experiment shows how laser detectors can be used to pick up conversations inside a room, by analyzing the vibrations of the glass windows.

This experiment was submitted by Ken Zetie, Head of Physics at St Paul's School in West London. He is on the editorial board of Physics Education and regularly contributes to Physics Review.