Why does the light from the lighthouse shine so far?

Because there is often a mirror behind the light source of a lighthouse.

Before 1800' s, a mirror was often installed behind the light source of the lighthouse to reflect the light and guide it to emit from multiple angles, but the light emitting distance was close and it was easily affected by weather conditions. By the early1820s, the French physicist Augustine Fresnel put forward the design concept of Fresnel lens, which can not only keep the parallel light formed by the refraction of convex lens, but also greatly reduce the weight of solid lens, thus reducing the manufacturing cost. By redirecting more light and adjusting the light to be parallel or focused, light can be emitted brighter and farther.