Model of a microscope using a car lamp

Class practical

Using a lamp to model the passage of rays through a microscope.

Apparatus and Materials

• For each student or group of students
• Plano-cylindrical lens, approximately + 13 D
• Plano-cylindrical lens, approximately + 7 D
• Triple slit
• Power supplies for lamp filaments

Health & Safety and Technical Notes

Read our standard health & safety guidance

The lamp is made from a car tail-light lamp. The lamp is designed to fit into a special base, but this is probably not worth the expense. A small wood block can be drilled with a hole to provide a push fit for the lamp. Two wires should be soldered to the contacts on the lamp and a third wire to the cap. The lamp should be pulled into the block with its axis horizontal so that the filaments are vertical. The two filaments act as two very close object points.

For the rays to be of approximately equal brightness, the 21 W filament should be run at 6 V and the 5 W filament at 12 V.

Procedure

1. Place the + 13D objective lens, with its plane side towards the lamp, about 12 cm from the filaments. This makes the images of the two filaments about 20 cm from the lens. A narrow aperture for the objective lens is needed for a clear image.
2. Place the + 7D eyepiece beyond this image, so that the final image appears to come from the object plane. It may help to set up the model if one of the filaments is switched off.
3. If the ray streaks are not clear, then the slits can be removed and the full cone rays traced along the bench.
4. The magnification can be measured by measuring the size of the object and the final image.

Teaching Notes

This experiment was safety-tested in January 2007