Question

34. In children's eyes, the lens is more flexible than in adults and may be fattened enough to focus on objects that are very close. This near point for one young girl is 8 cm, while her far point (the furthest point she can focus with her lens completely relaxed) is 2 m. The distance from her lens to her retina is 2.00cm a) Between what limits, measured in diopters, does the power of her lens-cornea com bination vary? b) Find the power of the lenses that an optician would prescribe so that she is able to focus on all distant objects. c) When she is wearing her glasses, what will be her new near point?

Answer 1

There will be two cases we need to consider in order to find out the limits of the lens For near pint v = -8 cm v = + 2 cm f = ? 1/v - 1/u = 1/f 1/2 - (- 1/8) = 1/f 1/f = 5/8 p = + 62.5 D For far point u = -200 cm v = + 2 cm 1/f = 1/2 - (-1/200) = 101/200 p = + 50.5 D The power range of lens cornea combination is + 50.5 D - + 62.5 D b) To focus all the objects the far point should be infinity u = infinity v = 2 cm f = 2 cm p = + 50 D now as the eye lens already has + 50.5 D we need to have a concave lens of - 0.5 D c) to calculate the new near point, let us calculate the object distance p = 62.5 D - 0.5 D = 62 D