Self-shadow and drop shadow

Why does a shadow form?

A shadow appears when an opaque object (one that does not let light through) is placed in the path of light rays coming from a source. As light travels in a straight line, it cannot go round the object: an unlit area forms behind it.

Two types of shadows

• The object’s own shadow: this is the part of the object itself that is not illuminated (for example, the hidden side of a ball).
• The cast shadow: this is the dark area projected onto a screen, a wall or the floor, caused by the object blocking the light.

Practical example

When you stand in the sun, your body casts a self-shadow (your back, if the sun is in front of you) and a cast shadow (the dark silhouette projected onto the ground). The lower the sun is on the horizon (in the morning or evening), the longer the cast shadow.

Drawing a cast shadow

To draw the cast shadow of an object, you draw the rays of light emanating from the light source, passing through the outermost points of the object, and extend these lines to the screen (the ground, for example). The area between these lines on the screen is the cast shadow.

Pitfall to avoid

Do not confuse a self-shadow with a cast shadow: one is on the object itself, the other is cast elsewhere. Another common mistake is to assume that the shadow always has the same shape as the object. In reality, its size and shape depend on the position of the light source and the angle of the rays.