I.

Introduction

Light, form of electromagnetic radiation similar to radiant heat, radio waves, and X rays. Light consists of extremely fast oscillations of an electromagnetic field in a particular range of frequencies that can be detected by the human eye. Different colour sensations are produced by light vibrating at different frequencies, ranging from about 4 × 10¹⁴ vibrations per second for red light to about 7.5 × 10¹⁴ vibrations per second for violet light. The visible spectrum of light is usually defined by its wavelength, ranging from the smallest visible wavelength for violet, about 40 millionths of a centimetre (16 millionths of an inch), to 75 millionths of a centimetre (about 30 millionths of an inch) for red. Higher frequencies, corresponding to shorter wavelengths, comprise ultraviolet radiation, and still higher frequencies are associated with X-rays. Lower frequencies, which are at longer wavelengths, are called infrared radiation, and still lower frequencies are characteristic of radio waves. Most light comes from electrons that vibrate at these high frequencies when heated to a high temperature. The higher the temperature, the greater the frequency of vibration and the bluer the light produced.