Light, shadows and how we see

Sources and reflectors

A light source makes its own light: the Sun, a lamp, a flame. The Moon and a mirror are not sources — they only reflect light made somewhere else. This distinction is asked about almost every year.

How we see

We see an object when light from a source bounces off it and enters our eyes. That is why nothing is visible in a completely dark room: there is no light to bounce. Eyes do not send light out.

Shadows

A shadow forms because light travels in straight lines and cannot bend round an opaque object. Move the object closer to the light and the shadow gets bigger, because the object blocks a wider spread of rays.

Three words for materials

Opaque blocks light completely; translucent lets some through but you cannot see clearly; transparent lets light through so you can see shapes clearly.