When it comes down to it, studying science is really just trying to figure out how the world around you works. So anything you can use to do that has to do with science. Mirrors and lenses both have to do with light, something that many scientists are really interested in.
Mirrors work by reflecting light.
When light hits the surface of a mirror, it bounces off, just like a ball bounced off the ground. Lenses are a little bit different - they let the light go right through them, but as the light goes through, it gets bent.