Ordinary light sources emit light in all directions. In order to make the emitted light propagate in one direction, it is necessary to install a certain concentrating device on the light source. For example, the headlights and searchlights of automobiles are equipped with reflecting mirrors with the function of concentrating light, so that the radiated light can converge and emit in one direction. Lasers emitted by lasers naturally emit in one direction. The divergence of the beam is very small, about 0.001 radians, close to parallel. In 1962, human first used laser to irradiate the moon. The distance between the earth and the moon is about 380,000 kilometers, but the laser spot on the surface of the moon was less than two kilometers. If the effect of focusing is very good, the seemingly parallel searchlight pillar will shoot at the moon and cover the whole moon according to its spot diameter.
2.Extremely high brightness
Before the invention of laser, the brightness of high-voltage pulsed xenon lamp in artificial light source was the highest, which was equal to the brightness of the sun. The laser brightness of ruby laser could exceed tens of billions of times that of xenon lamp. Laser has a high brightness, so it can illuminate long-distance objects. The illumination produced by the ruby laser beam on the moon is about 0.02 lux (unit of illumination). The color is bright red and the laser spot is clearly visible. If the strongest searchlight is used to illuminate the moon, the illumination is only about one-trillionth of Lux, and the human eye can not detect it at all. The main reason for the high brightness of laser is directional luminescence. A large number of photons are emitted in a very small space, and the energy density is naturally very high.
The color of light is determined by the wavelength (or frequency) of light. A certain wavelength corresponds to a certain color. The wavelength distribution of sunlight ranges from 0.76 micron to 0.4 micron, and the corresponding color ranges from red to purple, so the sunlight is not monochrome. The light source that emits a single color light is called a monochrome light source. It emits a single wavelength of light. For example, kr lamp
, helium lamp, neon lamp, hydrogen lamp and so on are monochrome light sources, which emit only one color of light. Although the wavelength of monochrome light source is single, it still has a certain range of distribution. If kr lamp only emits red light, monochrome is very good, known as the crown of monochrome. The range of wavelength distribution is still 0.00001 nanometer. Therefore, the red light emitted by krypton lamps, if it is carefully identified, it still contains dozens of kinds of red.