What's the difference between a lamp and a radio? We can hear a loud radio anywhere in the house, it doesn't matter much whether or not we turn our ears to it.

The picture shows how sound travels around a ball towards the ear. The sound doesn't carry much information about the form of location of the ball. Sound only gives a very limited sensation of form, and thus a limited sensation of space.

Here we have a lamp. The blue ball is an obstacle or the light of the lamp. As a result of the presence of the ball, the lamp is not directly visible. We can only see the light of the lamp indirectly, reflected by the green ball. Now a striking, almost magical phenomenon becomes apparent: as a result of the light meeting the balls the green ball has become visible, it got color. So where light touches an object, we see color being created. From the fact that the green ball is visible, but not the lamp, the onlooker can gather that the lamp must be hidden behind an unknown object: the blue ball. Lamp, balls and onlooker have a fixed relationship and together they create a sensation of form and space. One of the most important features of this sensation is color.