CD Scanner

Model CD scanner

Demonstration

Students apply their knowledge and understanding of wave superposition to an everyday application.

Apparatus and materials

• Wave transmitter and modulator, 3 cm
• Wave receiver, 3 cm
• Amplifier and loudspeaker
• Leads
• Model CD (see technical notes)

Technical notes

To make the model CD you need

·         Piece of chipboard or similar, approx 1.5 m x 20 cm (e.g. a shelf).

·         Pieces of plywood or similar, e.g. 0.75 cm thick (build up with card to get the correct thickness). Calculate l/4 from the wavelength of the microwave kit you use.

·         Aluminium foil or metallic spray paint.

Cut the plywood into pieces approx 8 cm x 4 cm.

Fix the plywood pieces in a line along the centre of the chipboard, so that there are spaces of about 10 cm between them.

Cover the surface of the chipboard and plywood with metal foil, or spray with metallic paint. 

Safety

Modern equipment using a solid state diode transmitter is safe. Older equipment using a klystron transmitter uses hazardous voltages. The connectors on the leads between the transmitter and the power supply MUST be shielded types to minimise 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.

Procedure

1.      Support the model CD as shown in the diagram. Ideally, mount it on dynamics trolleys so that it can easily be moved to and fro.

2.      To get the correct orientation of transmitter and receiver, arrange for a flat part of the CD surface to be in the beam of the transmitter and adjust the transmitter and receiver to get the strongest possible signal.

3.      Adjust the height of the 'CD' so that the 3 cm waves will be reflected from the plywood pieces.

Photo courtesy of Mike Vetterlein

4.       Slide the 'CD' to and fro so that the plywood pieces move in and out of the beam. Notice the change in intensity of the reflected signal. When a 'bump' is within the beam, the signal is weak, whereas when the beam is reflected from a flat section the signal is strong. 

Teaching notes

·          Students can be asked to discuss the way the reflected signal strength changes, and suggest an explanation based on their knowledge of waves and superposition. Alternatively, this demonstration could be used to introduce constructive and destructive superposition.

·         Information is encoded digitally onto the surface of a CD so that it can be read as a sequence of 'on/off' pulses.

The height of the bumps is one-quarter of the wavelength of the radiation. When a bump lies within the beam, some radiation is reflected from the bump and some from the surrounding area. There is a path difference of one-half a wavelength between these two sets of reflected waves. They combine at the receiver in antiphase, and the net result is a very weak signal. With no bump in the beam, the reflected waves are all in phase so the signal is strong.

In a real CD player, the CD surface is scanned with a laser emitting radiation of wavelength a few hundred nanometres. This model is scaled up by a factor of several hundred million.

·      A more realistic version of this model involves a rotating disc. Details are given on the Pearson website. Follow links to the Salters Horners Advanced Physics pages and click on 'free resources'.