A convex lens of focal length is cut horizontally into two equal halves along its principal axis. What is the focal length of each individual half-lens?
Answer and explanation
Correct answer: D.
- Step 1: The focal length of a lens is determined by the radii of curvature of its refracting surfaces and its refractive index: .
- Step 2: Cutting the lens horizontally along the principal axis leaves the curvature radii and and refractive index of each half completely unaltered.
- Step 3: Therefore, each half retains the identical focal length (only the light-gathering area is halved, reducing image brightness by ).
Answer
.
Why not the others: assumes doubling of power; is obtained when a lens is cut vertically perpendicular to the principal axis; Infinity implies flat glass.
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