A candle emitting light behaves in this way:
How it works - Basic
Fundamental Law: Light travels in a straight line
A candle emitting light behaves in this way:
Tracing out some of the light rays illustrates this point
Adding a light tight box blocks light from entering box
If we add a small hole at the center, some light can enter
We can trace light reflecting off of the rest of the candle in the same way
And form an image of the candle upsided down at the back of the box
If we now add film at the back of the box, a cap at the hole to control when light enters the box and exposes the light sensitive film, we have a camera.
That's it. This is a basic pinhole camera and all that is needed to make images.
The rest is refinement to a specific end.