Above: A living history group fires a World War II-era 25 pdr. field gun, producing both light and sound. (Scott Taylor)
The speed of both sound and light are fast... and it's a good thing too! Imagine walking into a dark room, turning on the light and then having to wait around until you can actually see anything. Or you're talking to your friends and you see their lips moving, but no sound is heard until some time later.
So exactly how fast are sound and light? According to the most recent measurements, sound travels in air at 331.3 m/s (767 mph), and light travels at a blistering 299.8 million m/s (670.6 million mph)!
It took researchers about 400 years to finally agree on these figures. Let's have a brief look at how this research got started.
Speed of sound
The first known attempt to measure the speed of sound was by Pierre Gassendi in 1635 who timed the delay between the powder flash of a distant cannon and the explosion that sounded later (the delay is similar to that between the flash of lightning and the clap of thunder). By the early 1700s, estimates for the speed of sound using cannon fire were within 0.5 m/s of modern-day values.
These experiments worked quite well because of the huge difference between the speed of sound and light. Trying to measure the speed of light was a completely different matter.
Did you know? There is nothing in the universe that is known to travel faster than light in a vacuum.
Speed of light
For centuries, scientists thought that the speed of light was infinite and could not be measured. Even today, it's difficult to imagine measuring something that's moving close to 300,000 km/s!
But in the 1760s, the Danish astronomer Ole Roemer discovered that the time for the moon Io to orbit Jupiter varied depending on how far Earth was from the planet. He reasoned that this could only be possible if it took light longer to reach Earth when it was furthest from the planet... that is, the speed of light is finite. A short time later, Christian Huygens used Roemer's data to calculate a speed which was within 25% of today's value.
Did you know? It takes about 4 years for light to travel from the nearest star to Earth.
In 1849 the first earth-based measurements were made by the French physicist Hippolyte Fizeau. He created a device to measure how long it took for a light beam to reflect off a mirror located 17.3 km away.
Thirteen years later another French physicist, Leon Foucault, modified Fizeau's apparatus and derived an estimate for the speed of light that was within 1% of the currently accepted value.
Scientists realized that they needed a way to measure speeds in gases other than air as well as in solids and liquids. Also, as technology advanced in such areas as telecommunications, precision and accuracy became increasingly important. This meant that experiments had to move into the laboratory.
So to improve their measurements, scientists took advantage of the fact that both sound and light move in waves.
Wave motion of sound and light
Sound in air is an example of a longitudinal wave where the air vibrates in a series of compressions and expansions (you can visualise how this looks by pushing and pulling one end of a Slinky back and forth). In contrast, light is a transverse wave where the wave oscillates at right angles to the direction of motion (e.g. flipping one end of a rope up and down).
Calculating the speed of motion
Both types of waves exhibit a frequency (the number of waves passing a fixed point in a given time period) and a wavelength (the distance between the crest of each wave). Therefore, precise measurements for the speed of sound and light can be made by measuring their wavelengths at a known frequency, where frequency is in units of hertz (cycles per seconds):
Wave speed (m/s) = frequency (Hz) × wavelength (m)
A Kundt's (or resonance) tube is often used by students in physics labs to learn how the speed of sound can be measured based on frequencies and wavelengths.
Standard values for the speed of sound and light
Did you know? The speed of sound increases at higher temperatures and is generally slowest in gases and fastest in solids.
Researchers at the Pennsylvania State College used an acoustic interferometer in 1942 to derive the first widely adopted value for the speed of sound in air (331.45 m/s) at 0 degrees C and an atmospheric pressure of 760 mm Hg. It was later modified to 331.29 m/s in 1984 by Canadian researcher Dr. George Wong who found a mathematical error in the original calculations.
The present day value for the speed of light is based on research at the National Bureau of Standards in Boulder Colorado in 1972. Using a helium-neon laser (i.e. a laser interferometer), they were able to get precise measures of the frequency and wavelength to calculate a value of 299,782.458 km/s in a vacuum.
Did you know? The speed of light in media other than a vacuum is corrected by using its refractive index (the ratio of the speed of light in a vacuum to the speed of light through that material).
The next time you're watching a movie or TV, think about this... you'd have to be a minimum of about 70 m (230 feet) from the screen before you might start seeing the picture and sound out of sync.
- How the speed of sound in air under standard conditions of temperature and pressure can be used to approximate speeds in other gases. Click here
- Mathematical formulas for calculating the speed of sound in solids, liquids and gases. Click here
- A brief overview of the refractive index. Click here
These manuals from Pasco Scientific explain the theory behind the use of a resonance tube and the experiments of Foucault to determine the speed of sound and light, respectively. They can be obtained free as a PDF from the web:
- H.C. Hardy, D. Telfair, and W.H. Pielemeier. 1942. The velocity of sound in air. J. Acoust. Soc. Am. 13, 226-233.
- K. M. Evenson, J. S. Wells, F. R. Petersen, B. L. Danielson, G. W. Day, R. L. Barger, and J. L. Hall. 1972. Speed of light from direct frequency and wavelength measurements of the methane-stabilized laser. Phys. Rev. Lett. 29, 1346-1349
- George S. K. Wong. 1986. Speed of sound in standard air. J. Acoust. Soc. Am. 79, 1359-1366.
- Yoshio. Saito. 2005. A discussion of Roemer's discovery concerning the speed of light. Association of Asia Pacific Physical Societies Bulletin. June 2005