In the history of science, the first applications of optical microscopes and telescopes to investigate nature mark the beginnings of new eras. Galileo Galilei used a telescope to see for the first time craters and mountains on a celestial body, the Moon, and also discovered the four largest satellites of Jupiter. With this he opened the field of optical astronomy. Robert Hooke and Antony van Leeuwenhoek used early optical microscopes to observe certain features of plant tissue that were called “cells,” and to observe microscopic organisms, such as bacteria and protozoans, thus marking the beginning of optical biology. The newly developed instrumentation enabled the observation of fascinating phenomena not directly accessible to human senses. Naturally, the question of whether the observed structures not detectable within the range of normal vision should be accepted as reality at all was raised. Today, we have accepted that, in modern physics, scientific proofs are veri-fied by indirect measurements, and the underlying laws have often been established on the basis of indirect observations. It seems that as modern science progresses it withholds more and more findings from our natural senses. In this context, the use of optical instrumentation excels among ways to study nature. This is due to the fact that because of our ability to perceive electromagnetic waves at optical frequencies our brain is used to the interpretation of phenomena associated with light, even if the structures that are observed are magnified a thousandfold. This intuitive understanding is among the most important features that make light and optical processes so attractive as a means to reveal physical laws and relationships.