The image of a candle in a plane mirror

In this activity, students investigate object and image distances for a plane reflective surface.

Learning outcome

Students understand that the image in a plane mirror (or other flat smooth surface) is as far behind the mirror as the object is in front.

Equipment

You will need a lighter or matches to light student candles. Each group of students will need:

• Two candles (eg tea lights)
• A flat transparent acrylic or glass screen to act as a ‘see-through‘ plane mirror (eg laboratory safety screen or an empty CD case)
• A holder or clamp to hold the transparent sheet vertical (alternatively an open CD case is self-supporting)
• A4 paper
• Pencil and ruler
• Beaker or drinks glass that is as wide and tall as one of the candles (optional)
• Smartphone (optional)

Procedure

Ask students to:

1. Stand the transparent screen in the middle of the A4 paper. Clamp or hold in place.
2. Place one candle about 10 cm away from the screen and light it. They should see an image of the flame in the screen.
3. Position the second unlit candle behind the screen. Move it back and forth until the image of the flame from the first candle seems to sit on the unlit wick of the second candle.
4. Mark positions of screen and candles by drawing around them. Remove both candles and screen.
5. On their paper, measure the distance from the lit and unlit candle positions to the screen position. These are their object and image distances respectively.
6. Repeat all steps to obtain a total of four object and image distances.

Teaching notes

The students should conclude that object and image distances are always the same. Provide a ray diagram to illustrate image formation and distances involved.

Extension

Students can set up and record the illusion of a candle burning under water by placing the unlit candle in a beaker and filling it with water.