Model of a microscope with ray streaks

Class practical

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

Apparatus and Materials

For each student or group of students

• Lamp and stand
• Lamp housing
• Triple slit
• Holder for slit or blocks to hold slit
• Plano-cylindrical lens, approximately + 7 D
• Plano-cylindrical lens, approximately + 13 D
• Power supply for lamps

Health & Safety and Technical Notes

Warn students that their lamps may trip the overload cutout on some supplies. Try connecting one lamp first and then adding the other one.

Read our standard health & safety guidance

Procedure

1. The lamp serves as an object. Position the lamp so that it is approximately 30 cm from the triple slit. Place the + 7D lens close to the triple slit, on the opposite side to the lamp, with the curved face of the lens facing away from the lamp.
2. Move the lamp up towards the lens until the image distance is two or three times the object distance.
3. Place the + 13D eyepiece lens beyond the focal plane of the objective lens, with its curved face towards the objective. Move the lamp quickly to and fro perpendicular to the axis, and watch the changing tilt of rays emerging from the eyepiece.

Teaching Notes

• This is a difficult model to arrange. It should be a class experiment, but many students will need help from the teacher.
• With a high-power objective it is difficult to get the lamp close enough, and to see the rays clearly enough, even when using the three slit comb. Using a + 7D objective and the + 13D eyepiece lens will make an instrument over 80 cm long.
• It may be possible to crowd two lamps close enough together to serve as top and bottom of the 'object'.
• If the rays are not clear, the comb can be removed and the full cone traced along the bench.

This experiment was safety-checked in March 2007