Purple lighting

A triangular prism dispersing a beam of white light. Light or visible light is electromagnetic radiation that can be perceived by the human eye. In physics, the term “light” may refer more broadly to electromagnetic radiation of any wavelength, whether visible or not. The main purple lighting of natural light on Earth is the Sun.

Historically, another important source of light for humans has been fire, from ancient campfires to modern kerosene lamps. The behavior of EMR depends on its wavelength. Higher frequencies have shorter wavelengths and lower frequencies have longer wavelengths. When EMR interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries.

There exist animals that are sensitive to various types of infrared, but not by means of quantum-absorption. Infrared sensing in snakes depends on a kind of natural thermal imaging, in which tiny packets of cellular water are raised in temperature by the infrared radiation. Above the range of visible light, ultraviolet light becomes invisible to humans, mostly because it is absorbed by the cornea below 360 nm and the internal lens below 400 nm. Plant growth is also affected by the colour spectrum of light, a process known as photomorphogenesis. The fixed value of the speed of light in SI units results from the fact that the metre is now defined in terms of the speed of light. All forms of electromagnetic radiation move at exactly this same speed in vacuum.

Different physicists have attempted to measure the speed of light throughout history. Galileo attempted to measure the speed of light in the seventeenth century. An early experiment to measure the speed of light was conducted by Ole Rømer, a Danish physicist, in 1676. Another more accurate measurement of the speed of light was performed in Europe by Hippolyte Fizeau in 1849.

Fizeau directed a beam of light at a mirror several kilometers away. Michelson conducted experiments on the speed of light from 1877 until his death in 1931. The effective velocity of light in various transparent substances containing ordinary matter, is less than in vacuum. The study of light and the interaction of light and matter is termed optics. The observation and study of optical phenomena such as rainbows and the aurora borealis offer many clues as to the nature of light. Due to refraction, the straw dipped in water appears bent and the ruler scale compressed when viewed from a shallow angle.