Class practical

Shows that image and object positions are symmetrical in a plane mirror.

Apparatus and Materials

For each student or group of students

• Board and sand tray if necessary
• Holder for mirror
• Plane mirror
• Candles, 2

Health & Safety and Technical Notes

Night-lights (with a suitably short mirror) are safer than candles. But if tall candles are to be used, the board should stand on a bed of sand in a metal tray.

Read our standard health & safety guidance

The lighted candles should be identical, stand upright and be less tall than the mirror. Failing candles, screws, pieces of chalk, or bulldog clips might be used as poor substitutes; pins should not be used as they are too small and it is difficult to keep them vertical.

Procedure

1. One candle is used as the object, the other as a decoy to catch the image.
2. Stand one candle in front of the mirror as the object. Place the second candle behind the mirror as the image catcher. Move the second candle about until it exactly replaces the virtual image as the eye is moved upwards.
3. View the arrangement from other directions to check the position of the image.

Teaching Notes

• Avoid the quicker no-parallax method here, because it does not teach the important idea of an image so well.
• An interesting extension is to ask students if they can light a candle in a beaker of water. Replace the mirror by a glass plate (e.g. the base of a ripple tank). Place a candle in a beaker of water behind the glass plate and a second candle of similar dimensions, an equal distance in front of the glass plate. The laboratory needs to be quite dark. Light the object candle. Its reflected image overlaps with the candle in the beaker of water, making the candle in the water appear to be lit.

This experiment was safety-tested in January 2007