The way that light is bent when entering a lens is called?

• A. Focal length
• B. Refraction angle
• C. Light path length
• D. Index of refraction

Answer: (D)

Bending of light when it enters a lens is governed by refraction that occurs because light changes speed as it moves between media with different optical densities. The index of refraction is the property that tells you how much the light slows down in the material; a larger index means greater slowing and, therefore, a greater change in direction according to Snell’s law (n1 sin theta1 = n2 sin theta2). In air-to-glass, the jump from about 1.0 to around 1.5 makes light bend toward the normal, producing the focusing effect of the lens. The index of refraction is the measure that determines how much bending occurs, whereas focal length is about where the rays converge, and the refraction angle is the bend itself, not the property that governs it.