Stage lights: physics and drama Teach article

Imagine sending music across the room by laser. Sounds impossible, doesn’t it? But Alessio Bernardelli’s students did just that – and then developed a play to explain the science behind it. Here’s how to do it.

To encourage our Year 12 students (ages 16-17) not to consider everyday technology as a ‘black box’ but to ask themselves how it works, my colleagues and I asked them to research, design and build a modulated laser, based on a cheap commercial laser pen. When they found a plausible design, they were astonished at how simple it was. Even some of the teachers had doubted that it would work.

In fact, it exceeded all our expectations. Not only did the modulated laser send an audio signal across the room, the music received by the light sensor and played on the speaker was loud and clear. Our students were amazed that they had made something that they hadn’t even known was possible. In fact, it is quite a simple process and can be used to explain a wide range of physics topics – as our students did in the next stage of their project.

What I like about our device is that it is very cheap and very visual. You could buy a similar device for about £250, but ours cost about £15 to make. Also, you can see exactly how our device is constructed, so that even the least scientifically minded can see how the circuit is connected.

Building the modulated laser

The input signal from the audio device is sent across a resistor. Because this signal is AC (alternating current), it causes variation in the voltage across the resistor, which in turn causes the overall current in the circuit to change with the same frequency as the input signal. This causes the intensity of light in the laser diode to change with the same frequency.

The laser beam, of varying intensity, is sent to a light sensor attached to a mini-amplifier. Because the number of photons reaching the light sensor changes with the frequency of the audio input signal, the current generated by the light sensor also changes with that frequency. This AC signal is sent to the mini-amplifier, causing the cone of the speaker to move – and we can hear the music.

The capacitor in the modulating circuit is essential to let the AC signal through and prevent the batteries’ DC (direct current) from feeding back into the audio device.

Materials

• Laser pen (at least 5 mW)
• Mini-amplifier
• The Radio Shack ones seem to be the best and cheapest^w1
• 3.5 mm mono jack lead (one jack plug at each end)
• 3 AAA batteries
• Battery holder for 3 AAA batteries
• Small light switch (press on / press off type)
• Solder and wire
• 2 crocodile clips
• 100 μF capacitor
• 10 Ω resistor
• Phototransistor or light sensor
• An audio device such as an mp3 or CD player (any audio device that uses a 3.5 mm jack lead)
• 2 stands and clamps

Assembly

The modulating circuit

1. Open the laser pen and remove the batteries.
2. Attach one crocodile clip to the battery spring inside the laser pen.
3. Attach the other crocodile clip to the case of the laser pen; do not let the clips touch each other.
   If the laser beam does not work when the circuit is complete, swap the crocodile clips on the spring and the case (the laser diode only works if the current is flowing in the right direction).
4. Solder a wire to connect one of the crocodile clips to one end of the battery holder.
5. Solder a wire to connect the other end of the battery holder to the resistor.
6. Solder a wire to connect the opposite end of the resistor to the switch and close the circuit by soldering a wire between the loose end of the switch and the remaining crocodile clip.
   This completes the DC part of your circuit, but to modulate the laser beam you must alter the voltage across the resistor, so that an AC will be generated and superimposed on the DC from the laser diode battery.

7. Cut the double jack lead in half. At one of the cut ends, strip the coating off the wire. (Save the other half of the lead for the receiver, see left).
8. You will see that the lead is made of thin copper wires wrapped around another wire covered in rubber. Gather all the thin copper wires, separating them from the rubber-covered central wire (you might need to strip the rubber off to expose the central wire).
9. Solder one of the two wires (either the bunch of thin copper wires or the central wire; it does not matter which) to one end of the capacitor.
10. Solder wires to connect the other end of the capacitor to the resistor.

11. To complete your modulated laser pen, solder the remaining wire from the jack lead to the opposite end of the resistor.
12. Using the jack plug, you can now connect your laser pen to the audio device.

The receiver

1. Strip the coating from the wires in the other jack lead, as in steps 7 and 8, above.
2. Solder one of the two wires (it does not matter which) to one end of the light sensor, and the other wire to the other end.
3. Use the jack lead, plug the light sensor into the mini-amplifier and the receiver is finished.

Operation

Point the laser beam at the light sensor, aligning them carefully using clamps and stands at both ends.

Switch on the audio device. The music will be played by the mini-amplifier – to which it is transferred via the laser. To make the demonstration even more dramatic, connect the output of the mini-amplifier to a more powerful speaker.

Communicating the physics

So we’d built a great gadget, but what were we going to do with it? My colleagues and I thought the students could design an interactive lecture show – but they came up with a much more creative idea. They decided to write a play based on James Bond, using situations in the play to explain the physics concepts behind our device.

Our Year-12 students wrote an amusing script, full of sound physics and with plenty of audience participation. It was aimed at Year-9 students (ages 13-14), to make them appreciate physics more – and encourage them to consider studying physics at school or even university.

During the course of the play, the older students explained concepts from the GCSE curriculum (ages 14-16), including signal modulation, the visible spectrum as a means of communication, optical fibres and the reflection of light, as well as some more advanced topics such as the photoelectric effect. The script, which can be downloaded^w2, is self-explanatory and shows clearly how the concepts were presented.

The students not only performed the play in our own school but also visited other schools in Wales. The audience was impressed by the quality of the play, the students’ knowledge and their ability to transmit it effectively to younger people. The younger students in the audience thought it was a fun way to learn physics and liked the fact that older pupils rather than teachers were showing them these concepts.

The Year-12 students’ attitudes also changed. One girl commented that it was only after she scripted and presented the photoelectric effect to younger students that she started to really understand the phenomenon. This is a common experience for teachers: it is only when we have to teach the topics that we start to understand them thoroughly ourselves. Partly as a result of this project, all of the Year-12 students involved chose to study physics or engineering at university.

The legacy of the project

The project started in June 2008 but its effects are still being felt. When we visited other schools, our play was not only for the Year-9 students, but also for their teachers. It demonstrated how much physics can be explained with our device; and because we presented them with their very own modulated laser pen, they could do similar demonstrations with their own classes. And of course, my colleagues at Croesyceiliog School^w3 continue to use the device in their own lessons.

The impact of the project also extended beyond our region of Wales. In October 2008, together with some of our students, we ran a hands-on workshop for teachers at the Welsh Physics Teachers’ Conference, organised by the UK’s Institute of Physics^w4.

In April 2009, Science Made Simple^w5 visited our school and incorporated aspects of our project into their new show for primary and secondary schools, taking our project even further afield – across England and Wales.

Timing

The project ran for a whole year, as part of the Rolls-Royce Science Prize 2008-09^w6, for which we were one of nine finalists. The research and design stage took a couple of weeks, and the device itself was built in a few days. The theatre phase – writing the script, rehearsing and giving the performances – took about two months. Over the course of the rest of the year, we collaborated with the Institute of Physics and Science Made Simple.

During school hours, we devoted about two months of one-hour lessons (two per week) to the project. The rest of the work was done after school.

Acknowledgements

Alessio Bernardelli led the project, but it would not have been possible without the help of Matt Smith and John Ivins, colleagues at Croesyceiliog School.

At the time of the project, Matt was an exceptional trainee teacher at Croesyceiliog School. He played an essential role in the design of the circuit, and solved many problems that arose during the course of the project. He is now the director of physics in an international school in Rome, Italy.

John Ivins was a key member of the team thanks to his rapport with the students. He is currently the acting head of physics at Croesyceiliog School in Cwmbran in Wales, UK.