Start with blue light: you can illuminate different things and make them appear bright or dim. Move the thing further away and it gets dimmer. Put a barrier in the way, casting a shadow over the thing, and it gets much dimmer. The brightness is set by how you arrange things in the space around the source of the photons. This space gets explored and in some places there get to be many photons, so it's bright, in others, fewer photons, so it's dimmer.
To make it dimmer, just ensure some photons don't make it – put something in their way. Just absorb all or some photons, just using a darkened slide.