How Speed Of Light Is Measured?

Speed of light is constant, travelling at 299,792.458 km per second to be exact. While this is common knowledge, have you ever wondered how scientists actually measure speed of light? Well, you will be surprised to know that it took humanity several centuries before it was able to accurately measure speed of light.

In earlier times, even prominent thinkers like Aristotle had the wrong belief that light is static and does not have movement. Some even believed that light’s speed was infinite. To clear any doubts, here’s a quick look at various attempts to measure speed of light and how it was finally determined.

Use of lanterns – In the 17th century, physicist Galileo attempted to measure speed of light by using lanterns. The basic idea was to uncover a covered lantern and try to measure any time delay when it was observed from a distance. However, this experiment was inconclusive, as the distance was way too short to detect any delay in the movement of light.

Movement of celestial objects – In 1676, Danish astronomer noted that the eclipses of Jupiter’s moons took place earlier when Earth was close to the planet. Vice-versa, it took longer when Earth was farther away. This led him to the finding that light has a finite speed. Based on his calculations, he defined speed of light at 220,000 km per second. This was 27% less as compared to the real speed of light.

Aberration of light – Under this method, the movement of earth around the sun is compared with the simultaneous movement of celestial bodies like stars to arrive at the speed of light. This method was used in eighteenth century by English astronomer James Bradley. According to his calculations, speed of light was 301,000 km/s.

Rotating mirror – Under this system, a beam of light is passed through a tunnel that is several miles long. In the middle is a rotating cogwheel and at the other end is a mirror. On careful observation, one can notice the position of the cogwheel at the time the light beam was sent and its return journey. The speed of light can be calculated with this system by inputting the rate of rotation of the wheel, number of teeth in the cogwheel and distance between the wheel and mirror. Using this method in 1926, American physicist Albert A. Michelson calculated speed of light at 299,796 km/s.

Laser interferometry – This is one of the most modern and accurate way of determining the speed of light. The basic idea is to split a beam of light and reflect it to two different mirrors located at different directions. The beams are then directed to a detector that measures interference. This is then used to calculate the speed of light.