Which of the following lenses will bend the light rays through largest angle?
- A.Lens with power +2.0 D
- B.Lens with power +2.5 D
- C.Lens with power -1.5 D
- D.Lens with power -2.0 D
Correct answer
Explanation
The correct answer is B, Lens with power +2.5 D. The power of a lens measures how strongly it bends light, and +2.5 D has the largest magnitude among the four options.
The power of a lens is the reciprocal of its focal length in metres, P = 1/f, and its unit is the dioptre (D). A short focal length means the lens bends rays sharply to bring them to a focus close by, so its power is high. The sign only tells the type of lens: a positive power means a convex (converging) lens and a negative power means a concave (diverging) lens. How much a lens bends light depends on the size of the number, not its sign. A +2.5 D lens has a focal length of 40 cm, the shortest here.
Option A is wrong because +2.0 D (focal length 50 cm) bends light less than +2.5 D. Option C is wrong because a magnitude of 1.5 D is the weakest of all. Option D is wrong because a magnitude of 2.0 D is still smaller than 2.5 D.
Exam tip: P = 1/f (in metres); bigger magnitude of power = stronger bending, sign shows convex (+) or concave (−).
Practice Questions
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- B.Martin Luther
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- D.Copernicus
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