Sound and light: energy carriers
The classic experiment to show that light can radiate across a vacuum, but sound requires a medium through which to travel.
Apparatus and materials
Clapper bell in round flask
Nichrome wire (26 SWG), reel of
Rubber bung to fit flask
Voltage supply, low, (typically 12 V at 6 A)
Health & Safety and Technical notes
Vacuum pumps are heavy and should be lifted on or off a trolley by two persons. Rotary vacuum pumps are reliable if maintained properly. (See CLEAPSS Laboratory Handbook for details.)
Round-bottom flasks are much less likely to implode than flat-bottomed or conical ones. However, safety screens should always be used to protect demonstrator and class.
Wire the clapper bell to a length of rubber tube and suspend it inside the flask. Do not use wire supports – these will transmit too much sound out of the flask and make the demonstration less impressive.
a Shake the flask so that the bell can be heard ringing.
Evacuate the flask and shake the bell again to demonstrate that the sound can no longer be heard.
Re-admit the air and the bell can be heard once more.
b Remove the bell and fix a coil of nichrome wire inside the flask. Connect the Bourdon gauge via a T-piece to record the pressure.
Connect the coil to the variable voltage supply. Turn up the voltage until the filament glows.
Pump out the air and show that the glow from the wire is still visible.
1 The main point of the demonstration is to show students that sound waves require a material medium for their transmission whereas light can be transmitted through a vacuum. As the air is evacuated, the sound of the bell becomes gradually quieter. With a good vacuum and careful suspension of the bell (see technical note), the sound will cease, even though the bell is still seen to be ringing. By contrast, students see the nichrome wire glowing with or without the air.
2 Point to the reading on the Bourdon gauge to convince students that air really is being removed from the flask. Alternatively, if you cannot obtain a Bourdon gauge, you could later open the flask with its mouth under water, and watch it fill up with water.
3 Energy from the Sun travels to Earth as electromagnetic waves. Some of it is visible light, but our eyes are not sensitive to most of the radiation. All of this is transmitted across empty space at a very high speed (3 x 108 m/s). There is no actual 'stuff' moving along with the motion of the light. The energy is carried by waves in which there is a variation in the electromagnetic field.
Sound also travels as waves, but sound waves need a medium in which to travel. The energy from a bell is transmitted across the air in the flask, through the glass itself, and through the surrounding air, as a longitudinal wave. The kinetic energy of the bell is carried by a sound wave through air. The wave causes vibrations in the ear. These in turn transmit signals to the brain.
4 When the air is pumped out, there can be no convection currents. The glow of the nichrome wire may be brighter than before, as the wire will be hotter.
5 The sound from the bell will grow fainter even without a vacuum, not because the few remaining molecules cannot carry a sound but because the bell does not have enough energy to move the molecules of air when they are so far apart. This is often referred to as a poor ‘impedance’ match between the bell and the thinner air.
6 A school-quality vacuum pump cannot produce a perfect vacuum. The reason that the bell becomes inaudible is more subtle than the explanation you will want to give to most students. With air pressure inside the flask just 1% of its normal value, some sound will reach the inner surface of the flask. The boundary will reflect almost all of the sound, because of the change in density and wave speed (air to glass). At advanced level you may explain this in terms of the difference of acoustic impedance in air and glass, and the intensity reflection coefficient. This also explains why a gel is used between an ultrasound probe and the patient's skin.
For more information, see the article Warning Bells by Frank Harris in the School Science Review, September 2005.
This experiment was safety-checked in November 2005.