Wavelength

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Wavelength

Wavelength is the distance between two points of the same phase in consecutive cycles on a periodic wave. Wavelength can be expressed in many units of distance, the most logical being the meter. However, because so many kinds of electromagnetic radiation are of very short wavelength some smaller units are necessary. Some of the most common units of measure for wavelength are the centimeter, the nanometer (1x10-9 m), and the angstrom (1x10-10 m). The angstrom is named for the Swedish physicist Anders Angstngström who used this unit as a basic unit of measurement for his studies of the solar spectrum in the mid-1800s. Visible light (for humans) has wavelengths ranging from roughly 400 nm to about 750 nm.

When light passes from one transparent material to another, with differing refractive indices, it is refracted or bent, the extent of which is determined by the wavelength of light. Violet light, light with a high frequency, is bent more than red light, light with a low frequency. The explanation for this phenomenon was put forth by Isaac Newton, an English mathematician, in 1666.

A wave will advance the distance of one wavelength in a time equal to one period of the vibration. Since all frequencies of electromagnetic radiation travel at the same speed in a vacuum it is possible to relate the wavelength to the frequency. The wavelength l is equal to the speed of light c divided by the frequency n, the number of cycles passing a given point per unit time, l = c/n. The frequency is usually denoted in units of s-1. The wave number s, another useful unit of measurement, is related to the wavelength in a reciprocal fashion, s = 1/l. The importance of the wave number became known when Rydberg noticed a periodicity when observing the spectra of elements with respect to wave number rather than wavelength. Unlike wavelength, wave number is usually expressed as 1/cm.

There are many distinct regions of the electromagnetic spectrum. The regions are divided according to wavelength. Radio are waves whose wavelengths are below 0.1 m. Microwaves are waves with length between 0.1 and 1.0x10-3 m. Infrared light ranges in wavelength from 1.0x10-3 to 750x10-9 m. Visible light falls next in the ordering and extends to 400x10-9 m. Ultraviolet light ranges from 400x10-9 to about 1.0x10-9 m. X rays range in wavelength from 1.0x10-9 to about 5.0x10-11 m. Gamma rays are the highest in energy and range from 5.0x10-11 m to shorter wavelengths. The shorter the wavelength, the higher the energy of the photons. The energy-wavelength relationship for electromagnetic radiation is: E = hc/l, where h is Planck's constant, c is the speed of light in a vacuum, and l is the wavelength.