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For a normal eye , the far point is at infinity and the near point of vision is about 25 cm . The cornea of the eye provides a converging power of about 40 D and the least converging power of the eye lens behind the cornea is about 20 D . From this rough data estimate the range of accomodation (i.e the range of converging power of the eye lens ) of a normal eye . |
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Answer» SOLUTION :The eye usesits least converging power for an object PLACED at infinity . Therefore, total converging power of the cornea =40 +20=60 D. Now, for object at infinity i.e ., at u=`oo` `v=f=1/P=1/(60)m=5/3cm ` For the IMAGE of an object placed at the near point, the focal length is `f=(1/v-1/u)^(-1)=(uv)/(u-v)=((-25)xx5/3)/((-25)-5/3)=(25)/(16)cm` `therefore` Converging power of the lens `=(100)/((25)/(16))=64 D` `therefore` Converging power of the eye lens =(64-40)=24 D Thus the range of accomodation of the eye lens is 20 D to 24 D. |
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