English: A square drawn in perspective.

The green point represents the vanishing point. Each side of the square recedes into the distance towards this vanishing point. The blue lines are lines drawn to determine where the back edge of the square is drawn. These lines are similar to the rays of light that travel from the viewer's eye to the back edge of the square. Where they hit the picture plane is also where the blue lines cross the side of the square.

The rays of light used to see an object are, in a perspective drawing, drawn on the drawing itself to determine where objects must be placed.