Solutions: Medical Option - the EYE


A defective eye has an unaided far point of 2.5 m and an unaided near point of 0.20 m. A correcting lens is used to produce an aided far point at infinity.


(i) Name the defect of vision affecting the eye.


(ii) State one possible cause of this defect of vision.

The eyeball is too long or the cornea is too curved/powerful.

(2 marks)

(b) Complete the three ray diagrams on below for the defective eye.

1st diagram: rays focused on retina
2nd diagram: rays focussed before retina
3rd diagram: symbol for the concave (diverging) lens, rays diverging from lens, they should appear to come from the point 2.5m away but if you do that they miss the cornea on the examboard's diagram rays (after diverging) focused on retina

(5 marks)


(i) Calculate the power of the correcting lens.

u = infinity so 1/u = 0

The image is on the same side of the lens as the onject - so it is virtual, making v negative

v = - 2.5 m

P = 1/v + 1/u

= - 1/2.5 + 0 = - 0.4 D

(ii) Calculate the aided near point when wearing the correcting lens.

v = - 0.20 m (S/he would see the closeup image at this point when wearing the spectacles)

P = 1/v + 1/u

- 0.40 = 1/u - 1/0.20

1/u = 1/0.20 - 0.40 = 4.6

u = 0.22 m

(4 marks)

(Total 11 marks)