In 1857, Irish scientist John Tyndall discovered that he could extinguish a candle flame with his voice. This was possible because sound waves are created from the energy of vibrating objects. They travel in waves created by high- and low-pressure variations that we experience as sound. The time interval between the peak of each sound wave passing a fixed point determines its frequency. Scientists have discovered that frequencies of 40- to 50-hertz are capable of extinguishing candle flames. Amaze your friends by performing a science project using sound waves to blow out a candle.

1

## Materials

To make a candle snuffer, you need a 2-liter plastic soft drink bottle, cling film, a rubber band that will fit tightly around the largest part of the bottle, scissors, a small candle in a holder and matches or a lighter.

2

## Procedure

Cut off the base of the bottle. Poke the sharp end of a pair of scissors through the plastic to make an opening for your scissors (get adult help, if necessary). The edge of the plastic can be sharp, so use caution as you cut. Stretch a piece of cling film tightly over the cut end of the bottle, leaving a small overhang. Secure the cling film around the bottle's circumference with a rubber band. Stand the candle on a table or stable flat surface, and then light the candle. Point the neck end of the bottle at the flame, about two-inches away from the flame. Sharply tap the cling film with your finger as if it is a drum. Listen for the sound you create and observe the candle's flame.

• Cut off the base of the bottle.
• Poke the sharp end of a pair of scissors through the plastic to make an opening for your scissors (get adult help, if necessary).
3

## Explanation

You created a sound wave when you tapped on the cling film. Air molecules near the cling film began to vibrate, causing other nearby air molecules to vibrate. The vibrations (sound waves) travelled through the bottle and out of the neck. This either disrupted the availability of oxygen that the candle needed to continue burning, or made the candle's flame mechanically unstable, depending on the frequency and intensity of the sound.

• You created a sound wave when you tapped on the cling film.
• Air molecules near the cling film began to vibrate, causing other nearby air molecules to vibrate.
4

## Going Further

Relight the candle and experiment to see how far away from the flame you can hold the neck of the bottle and still blow out the flame. Record your results. Hold the neck of the bottle in various positions, such as over the flame or below the flame, to see if you can still blow out the candle. When holding it over the flame, keep your hand and the bottle a safe distance from the flame. Record your results. Can you make any conclusions about the way sound waves travel?

• Relight the candle and experiment to see how far away from the flame you can hold the neck of the bottle and still blow out the flame.