[Visible] light is made of certain light frequencies and wave lengths in the form of electromagnetic radiation and which are visible to the human eye. The speed of light depends on the environment it passes through and on the frequency of the light itself. In an empty space, light travels in a straight line at 300,000 km/sec, which means it takes it about 8 minutes to travel the distance from the sun to the earth. This speed, which is almost unimaginable, was never before surpassed, although it is the subject of many physical experiments.

Sources of light (our most intensive source of light is the sun) send out electromagnetic radiation and light up their surroundings. All objects which come into contact with the light radiation are referred to as receivers of the light. The intensity of the light, which is the work executed by the lighting object, is measured in watts (W) or kilowatts (kW). A light source does not emit light in terms of points but rather as rays. However, with many small or weak sources of light, light does travel in the form of points.

Each object onto which light falls casts a shadow. Such casting of a shadow causes such phenomena as a lunar eclipse or a solar eclipse. A shadow is defined as the area behind an object where the object did not allow light to pass through and where the light was not able to reach as a result of this. Furthermore, there are different kinds of shadows. While only one type of shadow occurs from a single light source, a second type of shadow is created when there are two points of light. A total shadow is created in the area where light rays cannot reach from both light sources, where there is absolute darkness in this area of total shade. We refer to areas where light from at least one of the light sources reaches as a penumbra.

These effects can be very easily observed during an eclipse of the sun or the moon. An eclipse of the moon occurs when the earth is positioned between the sun and the moon and casts its shadow onto the moon. Whenever the moon finds itself in the penumbra of the earth, we see it as dark grey in colour, but we cannot see the portion of the moon which is in the earth's total shadow. On the other hand, an eclipse of the sun can be seen whenever the moon is positioned between the earth and the sun in such a way that it casts its shadow onto the surface of the earth. From our point of view, the moon looks dark but is surrounded by a ring of light.