Optical Instruments34525.14(a) When the child clearly sees objects at her far point ()max125 cmp=the lens-cornea combination has assumed a focal length suitable of forming the image on the retina ()2.00 cmq=. The thin lens equation gives the optical power under these conditions as 11111rs1.25 m0.020 0 min metersfpq==+=+50.8 diopte= +farPWhen the eye is focused ()2.00 cmq=on objects at her near point ()minp10.0 cm=the optical power of the lens-cornea combination is 1m0.020 0rin meters==+1110.100fpq+=160.0 diopm= +tersneaP(b) If the child is to see very distant objects ()p→ ∞clearly, her eyeglass lens must form an erect virtual image at the far point of her eye ()125 cmq= −. The optical power of the required lens is 11108001.25in metersfpq==+=+−−P10.m=diopters Since the power, and hence the focal length, of this lens is negative, it is
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