The light spreads in a straight line in the room. We represent the direction of propagation of light rays in the drawings as straight lines, which we call light rays .

A point light source is a light source in which light is emitted from only one point.

The lights emitted from several light sources merge and do not "interfere" with each other. If, for example, two identical lamps illuminate the paper from the same distance, we see the paper twice as bright.

We treat a light source of any shape and size as a sum of point light sources. The light produced by a light source is the sum of the lights of the point light sources that make it up. When depicting such light sources in the figure, we mark only the rays that are important for the performance of one or another specific task or purpose.

Light travels in a straight line. If none of the light rays of the light source reach an area, a full shadow is created there; if only some of the rays of the light source reach it, a penumbra is created there.

Since light spreads in a straight line, when constructing shadows, you can find the rays that separate the shadow area from the illuminated area - light cannot get to one side of such rays, because no straight line can be drawn there from the light source. Such rays thus define the shadow region and can be called "significant rays".