If a rigid gas-permeable lens is fitted steeper than the corneal curvature, the lacrimal lens power is which sign?

If a rigid gas-permeable lens is fitted steeper than the corneal curvature, the lacrimal lens power is which sign?

• A. Plus
• B. Minus
• C. Zero
• D. Uncertain

Answer: (A)

The tear film between the cornea and a rigid lens acts as a thin lens, and its power depends on how the lens curvature compares to the cornea. If the rigid lens is steeper than the cornea, the tear layer between them becomes more curved (toward the eye), which produces a converging, positive refractive effect. In this situation, the lacrimal (tear) lens adds plus power to the optical system. If the lens were flatter than the cornea, the tear lens would behave with negative (minus) power, and if it matched exactly, the tear lens power would be near zero.

The tear film between the cornea and a rigid lens acts as a thin lens, and its power depends on how the lens curvature compares to the cornea. If the rigid lens is steeper than the cornea, the tear layer between them becomes more curved (toward the eye), which produces a converging, positive refractive effect. In this situation, the lacrimal (tear) lens adds plus power to the optical system. If the lens were flatter than the cornea, the tear lens would behave with negative (minus) power, and if it matched exactly, the tear lens power would be near zero.