Candles are associated with celebrations and birthdays, but they have a long history in science too. The candela is the unit of luminous intensity and it replaces older units based on a standard candle. The fabled wreckers of the West Country were alleged to have used candle-powered lanterns to lure ships onto the rocks. Lace-makers used candles combined with water-filled glass globes, to concentrate the light.
Choose a selection of transparent bottles and fill them with water.
Investigate the effects on the light from a single candle flame. Children will find that the curved bottles act as a lens to focus the light. Use aluminium foil to shape reflectors to find the best combination of reflector and lens to produce the brightest possible light spot on a white card.
Next, devise a candle-powered lighthouse that gives flashes of light at definite intervals. Possible approaches include using open-and-close shutters, like Venetian blinds; pendulums that periodically block the light; rotating wheels that have holes cut in them to let the light through; and circular screens mounted on a turntable.
On those elusive days when the sun shines, it is possible to throw a shadow of a candle flame on to a white screen. Try a bright lamp as an alternative. You can distinguish different regions in the candle flame, using the shadow.
The central part of the flame, consisting of wax vapour, and the hot outer region will be clearly seen. Take a drinking straw and blow horizontally through the top of the flame. This produces a blue jet of flame, the same colour as an ordinary gas-cooker flame.
The extra oxygen provided by the straw improves the combustion of the wax vapour in the candle flame. The same effect is seen when opening and closing the air-hole on a Bunsen burner, the flame alternates between yellow and blue. The fact that exhaled air from the straw improves combustion will raise questions about its composition.
Children may be surprised that exhaled air contains sufficient unused oxygen to improve the flame and raise its temperature.