In daily life, light does not require explanation. It is an intuitive and obvious thing for any human being having the use of vision. The phenomenon becomes more complicated if we try to understand it from a physical point of view. Light is the most fundamental form of energy and the most essential for life on Earth. Our main source of light is the Sun, but we have also invented many artificial sources to produce it. The presentation describes the main stages in the evolution of light source technology. The last big step in mastering the generation of light is the invention of the laser in the middle of the 20th century. The laser and the transistor are the two greatest scientific inventions of the century. The possibility of producing intense and coherent light appeared when Albert Einstein formulated in 1917 the hypothesis of the existence of the phenomenon of stimulated emission. The prediction was verified in 1950 when Theodore Maiman succeeded in producing the first laser beam using a ruby crystal excited by a flash lamp.

The invention, although promising, did not seem very useful. However, in the following years, scientists and engineers developed techniques to accurately measure multiple physical quantities using a laser beam and to explore the properties of matter.

Nowadays lasers are used in all fields: industry, medicine, communications, etc. In the presentation, the generalized use of lasers is illustrated by some examples of applications of the femtosecond laser, a type of laser that the author contributed to develop during his research.