The combination of the cornea and the crystalline lens shown below serves as a converging lens. In a perfectly functioning eye, an image is projected at a fixed distance on the retina, which is approximately 2cm from the lens. What is the power of the converging lens when a person without any visual impairment stares at a distant object?
- 0.5 diopters
- 10 diopters
- 25 diopters
- 50 diopters
To answer this question, you must use the thin lens equation 1/f = 1/do + 1/di, where f is the focal length, do is the object distance, and di is the image distance. Since the power (P) of a lens is equal to 1/f, you can set P = 1/do + 1/di.
The question stem states that image distance is fixed and equal to a length of 2 cm. Since the question states that the observer stares at a distant object, we can assume that 1/do is negligible and equal to zero. Therefore, the equation simplifies to P = 1/di. Since power is measured in diopters, which has the units of m-1, you must convert 2 cm to 0.02 m and solve. Thus, the power of the lens is equal to 50 diopters, or answer choice D.
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