Active materials respond to incoming light in a way that provides much more design freedom.

The incoming light has to be a narrow strip - that is what the shiny pole in the web site above does.

The incoming light on the communications board suddenly blinked on.

However, on earth, with air filtering the incoming light, the solar spectrum changes.

The two images can be subtracted, leaving hopefully only the incoming light from the source.

The space available at prime focus is severely limited by the need to avoid obstructing the incoming light.

The electrons see a bigger picture, and interact with incoming light in a different way.

And with modern electronics, very small quantities of incoming light can be amplified.

To actually get an image where the film is, the lenses take the incoming light and focus it there.

That effect only happens if the incoming light is exactly parallel.