What happens to the focal length of a lens as its thickness increases?

• A. The focal length decreases
• B. The focal length stays the same
• C. The focal length typically increases
• D. The focal length varies randomly

Answer: (C)

When the thickness of a lens increases, the focal length typically increases due to the lens's geometric and optical properties. In a thicker lens, the curvature of the lens surfaces often leads to a greater optical power required to converge or diverge light. As a result, the light rays take a longer path before they converge at the focal point.

Moreover, the lens maker's equation plays a role in this relationship, where the focal length is influenced by both the curvature of the lens surfaces and the refractive index of the material. A thicker lens can mean more pronounced curvature and a potentially higher refractive index affecting how light is refracted.

Thus, in general, increasing lens thickness correlates with an increase in focal length, reinforcing why this answer is the correct choice.