| From: Zardoz ® | 04/11/2001
19:38:59 |
| Subject: re: COSMOLOGY FAQ | post id:
484076 |
| How do
they measure the speed of light? From: James R (Avatar) Some ways which have been used to measure the speed of light include: timing the eclipses of Jupiter's moons (there will be apparent delays caused by the transit time from Earth to Jupiter at different times of the year); reflecting light off a rapidly rotating mirror and measuring the angle of reflection; reflecting light from a diffraction grating of known periodicity and observing the diffracted orders. There are many ways to measure the speed of light. Diffraction measurements, rotating mirrors and experiments with optical or other electromagnetic cavities come to mind. The speed of light got to the point where it was known so accurately that the speed could be measured more accurately than the length of 1 metre was defined. Therefore, it was decided to define the metre in terms of the speed of light. This definition says that 1 metre is the distance light travels in exactly 1/299792458 seconds. Therefore the speed of light is now exactly 299792458 metres per second. Different colours travel at the same speed, but the frequency and wavelength of the light varies for different colours. In mediums other than a vacuum, the speeds are slightly different for different colours, since mediums tend to disperse different colours a little. From: B.C. The speed of light was first measured by the Danish astronomer Olaus Romer in 1675.He came to his conclusion when he was measuring the time between eclipses of one of Jupiter's moons. After taking countless measurements, he noticed that the time between eclipses became longer as earth and Jupiter moved further apart. He rightly concluded that the difference in time was because light from the moon had further to travel. He got a value of 140,000 mps [miles per second].A more precise value was obtained in 1862 by Focault, using rotating mirrors to deflect a beam of light. He achieved a closer value of 185,000 mps or 298,000 kms/sec. | |