While the wording is a bit awkward, the previous poster isn't wrong. Just, in radio, it's referred to as a reflector, not a mirror. Same principle, different area of technology.

EM is incredibly interesting especially since all data communication, with the exception of copper wires, is EM. Fiber optic is light, which we've established, is EM, and wifi is radio, which is also EM. Apart from the copper in your ethernet/DSL/Coax cable, it's all EM. It's fascinating to me that we use EM for so much, and fiber is considered the pinnacle of data connections, yet, light propagates slower through glass than radio propagates through the atmosphere, so technically, wifi can get a signal from A to B faster than fiber can.... and we put that stuff in our house.

All EM is at, or near, the speed of light. Glass, used in fiber, tends to slow the light down about 30% or so.... that's fascinating because the internet is largely fiber, and so the information for this or anything else on the internet is being delivered to your device at, or very near the speed of light.

Anyway, I'm off topic. I'm just a gigantic nerd about this stuff.