Speed:

The maximum wave speed ever measured is 299,792 kilometers (i.e., 186,282miles) per second in a vacuum. This is generally round off to 300,000 km/s or 3.00 x 10⁸ m/s. This is the recognizable speed of light, the absolute maximum speed with which everything can travel. (Recently, a few experiments have suggested that certain effect travel faster than this, though over long distances, 3.00 x 10⁸ m/s is the speed limit as far as we recognize.) Minor disturbances than light, in media humbler than the intergalactic space, travel slower. Even light itself moves all along more slowly than this in media other than the vacuum.

In air at sea level, sound waves push all along at around 335 m/s, or around 700 miles per hour (mi/h). This is termed as Mach 1.Whenever you speak to someone across a room, your voice travels at Mach 1. The sound in air propagates at Mach 1 no matter what the frequency and no matter how strong it is. The precise figure differs a little, depending on the altitude, the temperature, and the relative humidity, though 335 m/s is a fine number to keep in mind.

Electromagnetic waves do not forever push the absolute cosmic speed limit. In glass or under water, the light waves propagate at appreciably less than 3.00 x 10⁸ m/s. The radio waves slow down whenever they pass via Earth's ionosphere. Such variations in speed affect the wavelength, even when the frequency stays constant.

PROBLEM:

One nanometer (i.e., 1 nm) is 10^-9 m. Assume that a light beam has a wavelength of 500 nm in free space and then come into a new medium where the speed of light is only 2.00 x 10⁸ m/s. The frequency does not change. What occurs to the wavelength?

SOLUTION:

Note that the formula below defines the wavelength in terms of frequency and speed:

λ = c/f

The speed is cut to 200/300 of its original value. Thus, the wavelength is also cut to 200/300 of its original value. The wavelength in the new medium is 500 nm x 200/300, or 333 nm.