# Rays of light

Experimentally can check how the light rays change direction at the transition, such as from air to water. Take the opaque metal or porcelain mug. At its bottom, place a stack of coins 5-6. Without taking his eyes off the coins, pushes your head until the edge of the mug do not close them by you. Without changing the position of the head, slowly pour in a cup of water. Rays of light will enter the eye and coins will become visible again. Figure explains this phenomenon. Rays reflected from coins at the outlet of the water refracted and fall into the eye. The same coins and the bottom mug, on a direct continuation of the rays seem to be raised.

Rays of light is "doing" this constantly. Take a glass, fill it with water, dip a spoon into it. You will see that in the place where the spoon into the water, it seems to be refracted, broken. In fact, the light rays are refracted (see figure), and this phenomenon is called refraction.

The concept of light rays is an approximation of geometric optics. This definition implies that the course of the light beam is independent of the transverse dimensions of the beam of light. In fact, the narrow light beam does not extend in a single direction, and has a finite angular distribution. However, in cases where the characteristic transverse dimensions of the light beams are sufficiently large compared with the wavelength of light can be neglected ray beam divergence and assume that it spreads in a single direction, along the light beam. Generally, it is believed that all the properties of the medium through which the light rays pass vary over distances of the order of the wavelength of light is very weak. Therefore, if the properties of the medium do not depend on coordinates (i.e. if the medium is uniform), the light beams are straight.

What is a beam of light rays? It is a lot of family of light rays. The transverse dimensions of the beam does not have to stay the same, because in general the different light rays are not parallel to each other. An important case of beams of light rays is homocentric beams, i.e. such bundles, all rays which intersect at any point in space. Such light beams may be formally derived from a point light source by means of an ideal lens. Not homocentric beams do not converge at a single point in space.