Kids Research Express

. Monochromatic light, or light of one color, has several characteristics that can be measured. As discussed in the section on electromagnetic waves, the length of light waves is measured in meters, and the frequency of light waves is measured in hertz. The wavelength can be measured with interferometers, and the frequency determined from the wavelength and a measurement of the velocity of light in meters per second. Monochromatic light also has a well-defined polarization that can be measured using devices called polarimeters. Sometimes the direction of scattered light is also an important quantity to measure. When light is considered as a source of illumination for human eyes, its intensity, or brightness, is measured in units that are based on a modernized version of the perceived brightness of a candle. These units include the rate of energy flow in light, which, for monochromatic light traveling in a single direction, is determined by the rate of flow of photons. The rate of energ

. B. Speed of Light Scientists have defined the speed of light in a vacuum to be exactly 299,792,458 meters per second (about 186,000 miles per second). This definition is possible because since 1983, scientists have known the distance light travels in one second more accurately than the definition of the standard meter. Therefore, in 1983, scientists defined the meter as 1/299,792,458, the distance light travels through a vacuum in one second. This precise measurement is the latest step in a long history of measurement, beginning in the early 1600s with an unsuccessful attempt by Italian scientist Galileo to measure the speed of lantern light from one hilltop to another. The first successful measurements of the speed of light were astronomical. In 1676 Danish astronomer Olaus Roemer noticed a delay in the eclipse of a moon of Jupiter when it was viewed from the far side as compared with the near side of Earth’s orbit. Assuming the delay was the travel time of light across Earth’s orbi