## Related questions with answers

The eyes of amphibians such as frogs have a much flatter cornea but a more strongly curved (almost spherical) lens than do the eyes of air-dwelling mammals. In mammalian eyes, the shape (and therefore the focal length) of the lens changes to enable the eye to focus at different distances. In amphibian eyes, the shape of the lens doesn’t change. Amphibians focus on objects at different distances by using specialized muscles to move the lens closer to or farther from the retina, like the focusing mechanism of a camera. In air, most frogs are nearsighted; correcting the distance vision of a typical frog in air would require contact lenses with a power of about –6.0 D. What is the farthest distance at which a typical “nearsighted” frog can see clearly in air? (a) 12 m; (b) 6.0 m; (c) 80 cm; (d) 17 cm.

Solution

VerifiedThe power of the lens is given by

$P = \dfrac{1}{s} + \dfrac{1}{s'}$

Now, we solve this equation for $s$ and plug the values for $P = 6 \mathrm{~D} = 6 \mathrm{~m^{-1}}$ and $s' = \infty$

$\begin{gather*} P = \dfrac{1}{s} + \dfrac{1}{s'}\\ \dfrac{1}{s} = P - \dfrac{1}{s'}\\ \dfrac{1}{s} = 6 \mathrm{~m^{-1}} - \dfrac{1}{\infty}\\ \end{gather*}$

Hence, $s = 0.17$ m = 17 cm. The correct answer is $\textbf{ (d) 17 cm}$

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