The Lighthouse Lens Effect · AI Generator #GEN-456F3730
Physics · optics
Problem
A lighthouse uses a large Fresnel lens to focus light from a lamp into a parallel beam. The lamp can be modeled as a point source located 12.0cm behind the lens. When a ship's captain looks at the lighthouse from far away, she sees a bright parallel beam of light.
However, when thick fog rolls in, water droplets condense on the lens surface, effectively changing its focal length. The captain notices that the light now appears to come from a virtual point source located 36.0cm in front of the lens (on the same side as the captain).
What is the focal length of the lens after the water droplets have condensed on its surface?