Lighthouses frequently magnify the light from the source to give extra brightness. The principle is simply to collect light that would otherwise be wasted (remember, light is emitted from a source in all directions) and concentrate it into a bright beam or parallel rays of light. Let's suppose we have a light source emitting 100 watts every second. It could be that only 5% of it (5 watts) travels off in the direction of the mariner's eye. If we can gather all 100 watts into a single beam and focus that beam in the direction of the mariner, then we could have a light that was 20 times brighter and could be seen at a greater distance.
To achieve this, optics are used which are built from complex arrangements of lenses and prisms. However, the first method used involved highly polished metal reflectors, in the shape of a parabola. You will have seen this often in such places as electric fires, car headlights and battery torch lights. Copper was frequently used for these reflectors because, being more corrosion resistant than iron-based alloys, it is easy to keep clean and shiny.
When glass lenses and prisms are used, rather different principles are involved. The polished and shaped pieces of glass can be used to change the direction of the light rays, lenses changing the direction by a little and prisms changing it by a lot. Of course, any material transparent to light could be used, but traditionally glass has been the material of choice. The two main properties involved are reflection and refraction of light. In simple terms, we can think of the light passing through the material and finding atoms and molecules of the solid "in the way". This results in both a slowing down and a change of direction of the light rays.