The average total power, front and back, of the cornea is how many diopters?

• A. 30 D
• B. 50 D
• C. 42 D
• D. 60 D

Answer: (C)

The cornea provides most of the eye’s refractive power, and its two surfaces work together like a thick lens. The front surface has a strong converging power, while the back surface has a small negative power. When you combine these with the thickness of the cornea and its refractive index, the net positive power ends up around 42 diopters. This is the standard average value used to describe the cornea’s total refractive contribution. Other options are farther from this typical figure: 30 D underestimates the cornea’s power, while 50 D or 60 D would imply a stronger net power than is normally observed.