Light travels in a straight line. We will represent the light coming from a source as a collection of lines representing the paths light takes, and we will call one of these lines a light beam. Later on, we will meet some exceptions and corrections to this picture, but we can understand most of what we observe from this simple representation.

Light beams start from every object that is giving off light or reflecting light, and travel in straight lines in all directions. To simplify our optics investigations, we turn off as many lights as possible, and have just one bright light. To simplify still more, this light will be small and far away, so that we don't have to distinguish between "light coming from the right side" and "light coming from the left side" of the object.

Here are some examples of suitable light sources:

Object	Size	Distance
Star	about 1,000,000 km	more than 10,000,000,000,000 km
Sun	1,390,000 km	149,000,000 km
Street lamp	1 foot	300 feet
Light bulb	2 inches	200 inches

The important point is that the distance is always large compared to the size. Then a picture of the light beams will always look the same:

The beams diverge from the source, which is almost a point.
This picture becomes simpler again if the light source is at a distance that is large compared to the region we are studying, because then the light beams that reach the object are all travelling in (almost) the same direction, and so are (almost) parallel to each other. For example, we can pretend that all the light beams from the sun are parallel to each other, when we are talking about a little region like the earth itself.

On the scale of this picture, the sun is 2000 feet to the left, and about 20 feet wide.

The beams are actually diverging from each other, just like in the first picture, but the difference is too small to see here. A more serious error in this picture is that the sun is not a point source. Beams coming from the top and the bottom edges of the sun are going in slightly different directions (it would be just barely visible in this picture). This is why sun shadows are slightly fuzzy at the edges.
Shadows