During the 1800's German mathematician Friedrish Gauss established a one inch square mirror could transmit sunlight seven miles. That led to the creation of the heliograph system of military signaling with mirrors.
In any event, this brings us to the advisability of carrying one signal mirror for every person on a wilderness trip. Not just "one" for the entire party. They should also be carried on boats.
Plus, you can use them to shave with.
These are idiot-proof, and are based on the concept of catching the attention of potential rescuers by flashing sunlight at them. The better ones have a hole in the middle and an aiming spot, but using one isn't exactly rocket science. They have to be moved from side to side to cause multiple flashes in the eye of the observor.