What is light, and how does it travel?

Light: a source of information about the world

Light is what enables us to see the objects around us. It is produced by primary sources (the Sun, a light bulb, a flame, a screen) which emit their own light. Objects that do not produce light, such as a table or a book, are secondary sources: we can only see them because they reflect some of the light they receive.

Propagation in a straight line

In a transparent and homogeneous medium (such as air or a vacuum), light travels in a straight line. This path is represented by a light ray, drawn as an arrow. It is this principle that explains why we can cast sharp shadows: an object in the opposite direction blocks the straight path of the light.

Practical example

When you switch on a torch in a dark room, the beam of light travels in a straight line towards the wall. If you hold your hand in front of it, a sharp shadow appears because the light cannot get round the obstacle.

Speed of light

Light travels very fast: around 300,000 km/s in a vacuum. At this speed, it circles the Earth in just over a tenth of a second.

A classic trap

Note: an object can be the primary source of light for one colour and invisible for another. But remember above all that without light, no object is visible, not even white or shiny objects.