Light sources

Light-emitting objects and illuminated objects

In order for us to see an object, light must reach our eyes. There are two categories of objects:

• Light-emitting objects (or primary sources): these produce their own light. Examples: the Sun, a flame, a lit lamp, a mobile phone screen.
• Illuminated objects (or secondary sources): these do not produce light, but reflect (scatter) the light they receive from a primary source. Examples: the Moon, a table, a book, your face.

A telling example: the Moon

The Moon does not shine on its own! It only appears bright to us at night because it reflects the light it receives from the Sun back towards Earth. If the Sun were to go out, the Moon would become invisible.

Summary table

Object	Type	Why we can see it
Sun	Primary source	It emits its own light
Lit lamp	Primary source	It emits its own light
Moon	Secondary source	It reflects the Sun’s light
A chair	Secondary source	It reflects ambient light

Common misconception

Many pupils think that the Moon ‘shines’ like the Sun: this is incorrect; it merely reflects sunlight. Another common mistake is to believe that a black object in total darkness can still be seen a little. In reality, without any light source, no object is visible, not even the lightest ones.