Question

Tiny beads of transparent material, usually less than a millimeter but still hundreds of wavelengths in diameter and made of glass for durability, are used in road and airport runway markings, and also in road signs. They perform best when the index of refraction of the material is close to 2. Why? Pick those answers that apply.

The transmission optics of a sphere cannot be explained with the thin lens equation

When n=2, all of the light reflects from the front surface. This makes the exposed bead a tiny convex mirror that focuses light back toward the observer.

The wave properties of light cause interference that makes the beads brighter when they are smaller than a millimeter in diameter.

When n=2, refraction at the first surface light encounters will focus it on the back surface. It will reflect at the back side like a mirror, and follow a similar path back out of the drop to "retro reflect" toward the source of light. It is the back surface that acts like a directional mirror.

Answer 1