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 opaque object 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: remember that without light, no object is visible, not even white or shiny objects.