The formation of shadows

The formation of the shadow, understood as the formation of a region devoid of light, is a consequence of the principle of rectilinear propagation of light.

Imagine an object of very small dimensions that emits light (a household lamp seen at a great distance). When light emitted by an object is the only source in a certain region of space, then an object at a certain height from the ground will cast a shadow on it.

This is because light upon finding the object will be prevented from proceeding, producing a region in which there is no light (the shadow). The other rays as they propagate through the space in a straight line will hit the floor or other object creating lighted and lightless regions (where there is shadow).

If the light source is extensive (not point), the most common case, then we will have regions not reached by light rays (shadow regions) and regions hit by some (but not all) light rays. These regions, of varying degrees according to the amount of light, are the regions of gloom.

Consider a spherical body constituting an obstacle to the propagation of light placed between the light source and a screen (a wall, for example). The shadow region in the spherical body and the shadow itself. The shadow region between the spherical body and the bulkhead is shaped like a cone and is therefore known as shadow cone. In the screen forms the shadow, or projected shadow.

In the case of an extended source, and assuming an equally spherical source, one obtains a shadow of its own on the spherical object located between the source and the bulkhead, a shadow cast on the bulkhead (region in the non-light bulkhead) and a gloom projected onto the bulkhead. The gloom is partially illuminated. The partially illuminated region between the spherical body and the bulkhead is the penumbra cone.


The previous cases, where we analyze the shadow and penumbra regions of spherical bodies and sources is important to understand the phenomenon of eclipses. It is a natural phenomenon that happens relatively often. The last recorded total eclipse of the sun occurred in 1999. Since the sun, moon, and earth are spherical bodies, the above considerations of shadow and gloom are worthwhile.

The eclipse of the sun occurs when the moon comes between the sun and the earth. The sun is eclipsed by the moon.

We call it total eclipse of the sun that situation in which some regions of the earth enter the shadow of the moon (shadow region). The regions entering the moon's dim cone perceive a partial eclipse (as they are in the moon's dimness).

There may still be another type of solar eclipse: the annular eclipse. In this type of eclipse a certain region of the earth (and its inhabitants) enters the extension of the moon's shadow cone. As a result, these regions will be exposed only to light from the peripheral part of the sun. The central part is naturally eclipsed by the moon. In this case, we have the annular eclipse of the sun. Because these regions are in the dimness of the moon, this type of eclipse is partial.

The situation that distinguishes between the two types of eclipse is the relative distance between the sun, the earth and the moon. These distances may vary enough to cause both types of eclipses.

The eclipse of the moon occurs when the earth comes between the sun and the moon. In this case, the moon first enters the earth's dim cone and then the earth's shadow region.