Science Articles

Comparing the Speeds of Sound and Light

Light and sound are natural phenomena that have fascinated scientists since ancient times. Their varying speeds too have been subject to painstaking hours of experimentation, exploration and enquiry, before scientists were able to obtain concrete information about it.

Light and sound are natural phenomena that have fascinated scientists since ancient times. Their varying speeds too have been subject to painstaking hours of experimentation, exploration and enquiry, before scientists were able to obtain concrete information about it.

Both, light and sound, are physical entities that travel in waves. Earlier, light was thought of as a collection of particles. However, it was later proved that light travels in waves. Einstein proved that both theories- wave theory and particle theory- are consistent with the nature of light. Light is thus said to exhibit a dual nature.

There are some important ways in which the speed of sound differs from the speed of light. They are as follows-

  1. Light waves can travel in vacuum, whereas sound waves cannot travel in vacuum.
  2. The speed of light waves in air is 3 x m/s, while that of sound waves in air is 330 m/s at 0. This means that the speed of light is a million times greater than the speed the speed of sound in air. In fact, the speed of light is the greatest velocity known to man.

Speed of light is a very important concept in modern physics. It is considered to be the only absolute parameter in the universe. According to the theory of relativity, the speed of light is the maximum velocity any object can hypothetically obtain. No particle can travel faster than the speed of light.

  1. The speed of light waves decreases in an optically denser medium. For e.g., the speed of light in water is 2.25 x m/s, while in glass, it is 2 x  m/s. The speed of sound waves is more in solids, less in liquids and lesser in gases. For e.g., speed of sound in steel is 5100 m/s, in water it is 1450 m/s, while in air, it is 330 m/s.
  2. Light waves are transverse electromagnetic waves while sound waves are longitudinal mechanical waves.

Some phenomena that occur as a result of the vast difference between the speeds of sound and light-

  1. When there is lightning in the sky, the bright light from the lightning is seen earlier whereas the sound of the thunder is heard a few seconds later, even though both are simultaneously produced in the sky.

The reason is that light takes negligible time to reach us from the spot that the lightning occurs, while sound reaches us later, because speed of light is much greater than the speed of sound.

  1. When the starter is fired in an athletic race, spectators are able to see the smoke and light of the starter gun faster, while they hear the starter sound a bit later.
  2. When a batsman hits a shot in the game of cricket, the spectators sitting in the stadium see the shot being hit faster, while they hear the sound of the shot a little later.

Thus, the speeds of sound and light make for very interesting topics in the study of Physics. Scientists have made use of these speeds to breach new frontiers in the study of the Universe, relativity, quantum mechanics and our world.