Since ancient times, the rainbow has attracted close human attention. In the Bible, she appears as a sign given by God as a sign of forgiveness and reconciliation with people. The English philosopher and naturalist Roger Bacon (circa 1214-1292) carefully examined the phenomenon of the rainbow in his major work, The Great Work. He believed that the colors of the rainbow were a subjective phenomenon caused by the moisture in the eye.
The first theory of the rainbow was given in 1637 by the French philosopher and mathematician René Descartes (1596-1650). A more accurate theory was developed in 1836 by the English astronomer George Erie (1801-1892). His theory is based on the calculation of the phenomena of diffraction and interference accompanying the meeting of the sun's rays with the lattice formed by raindrops.
A rainbow occurs when sunlight is refracted by rain or fog water droplets floating in the atmosphere. These droplets deflect different colors of light differently (the refractive index of water for longer wavelength (red) light is less than for short wavelength (violet)
For an observer on the ground, a rainbow usually looks like an arc, a part of a circle, and the higher the point of view of the observer, the fuller the rainbow (from a mountain or an airplane, you can see the full circle). The center of the rainbow arc is in the direction of the straight line passing through the solar disk and the observer's eye, that is, at the point opposite to the Sun. A rainbow arc is a portion of a circle around that point with a radius of 42 degrees.
The sequence of colors in a rainbow is the same as in the solar spectrum, and usually red is located along the outer edge, and violet along the inner edge. From the side of the inner edge, secondary color arcs are sometimes visible adjacent to the main rainbow. The visible part of the arc is determined by the position of the Sun: when it is on the horizon, the rainbow looks like a semicircle, as the Sun rises, the visible part of the arc decreases, and when the Sun is 43 degrees high, the rainbow disappears.
A phenomenon similar to a rainbow can be seen in the spray of fountains, waterfalls. The appearance of a lunar rainbow and a rainbow from artificial light sources is possible. A second rainbow is often observed with an angular radius of about 52 degrees and reversed colors.