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Solutions: Medical Option - the EYE

Q9.

A defective eye has an unaided near point at 0.65 m and an unaided far point at infinity.

You can tell from this statement that the person has long sight and you will therefore know that correction will be from a convex lens. This background info will help you know whether your calculation values are reasonable - you will expect to get a positive lens value

Calculate

(i) the power of the correcting lens needed to allow the eye to see clearly an object 0.25 m from the eye,

The image of an object at 0.25 m from the eye needs to be formed 0.65 m from it. The image will be virtual as it is upright and on the same side of the lens as the object.

v = - 0.65 m

u = 0.25 m

P = 1/f = 1/v + 1/u

= -1/0.65 + 1/0.25 = 2.46

= + 2.5 D

When calculating lens powers it is useful to put a + or - number in from of your answer - to make you think about it!

(ii) the furthest distance from the eye that an object can be seen clearly when the correcting lens is used.

The furthest distance the image can be formed is at infinity for the eye - therefore 1/v will be zero..

P = 1/u + 1/v

2.46 = 1/u + 0

(always work to one more sig fig than final answers need to be quoted in!)

u = 0.41 m

(Total 3 marks)