In the last centuries, physics has studied the phenomenon of \"light\" and revealed its secret: light is the small visible part of electromagnetic radiation. This electromagnetic radiation consists of oscillating energy units (quanta) and is sent in waves from a light source. It takes a certain time from the place of its origin to the eye of the observer. Each wavelength is determined by a color impression. The spectrum of sunlight shows a continuous transition: from short-wave violet through blue, green, orange to long-wave red. Outside this range, the human eye cannot \"see" radiation; gamma, X-ray, UV and infrared rays are not visible. The time it takes for light to travel from the object to the observer's eye is the speed of light. In 1850, the French physicist Leon Foucault developed the so-called rotating mirror method. It made it possible to determine the precise value of the speed of light: 2.98 x 108 m/s. For the speed of light in empty space and in air, the rounded-up value of 3 x 108 m/s - i.e. 300,000 kilometers per second - is generally used. The speed of light is thus the highest known speed. Light needs about 1.3 seconds from the moon to the earth. In contrast, the light of the sun, which is 150 million kilometers away from the earth, is already 81/3 minutes on the way. From the fixed star Alpha in the Centaurus it needs already 4.3 years. If it would explode today - we would know it only in some years.