The virtual image in a plane mirror

Class practical

Identifying that the image in a plane mirror is virtual.

Apparatus and Materials

For each student or group of students

• Large sheet of white paper (A3)
• Plane mirror
• Ray box or lamp (12 V 24 - 48 W)
• Low voltage power supply for ray box
• Multi-slit screen (comb)
• Barriers
• Holder for mirror

Health & Safety and Technical Notes

Read our standard health & safety guidance

Many ray boxes of traditional design become very hot after a lesson of use. Warn the class and provide them with heat-proof gloves or cloths if they need to handle the ray box when still hot.

Procedure

1. Place the plane mirror upright on the paper and arrange the lamp, multiple slit screen and barriers so that a fairly narrow fan of rays from the lamp hits the mirror. Move the lamp quite close to the multiple slits, so that the virtual image of the lamp will be somewhere on the actual paper.
2. Ask students to 'look along the reflected rays to see where they come from' and to look down on the pattern from above, and see where the reflected rays seem to come from.

Teaching Notes

• Encourage faster students to try to put some marking device at the place the rays seem to come from, such as a large pin stuck on the paper or a small block of wood.
• The crossover of the rays in the reflection can be quite confusing to sort out at some angles.
• Use the name virtual image and ask students where it is. Able students will be able to trace the rays back carefully towards the virtual image, showing that it is perpendicularly as far behind the mirror as the object is in front.
• It could be useful to bring out the ripple tank to show circular ripples being reflected from a barrier as in:

  Reflection of a circular pulse by a barrier

This experiment was safety-tested in January 2007