Solutions: Medical Option - the EYE

Q2.

(a)

(i) Draw a labelled ray diagram to show how a normal eye forms an image of a near point object on the optical axis of the eye. The diagram should show the path of two rays from the object and should indicate clearly the places where refraction occurs.

refraction at cornea and lens - lens should have indication that refraction occurs at both surfaces
correct rays showing focus at the retina

(ii) Explain why most of the refraction occurs at one of these places.

The greatest refraction occurs at the air/cornea boundary
because greatest difference in refractive index occurs at the air/cornea boundary
(4 marks)

(b)

Draw another labelled ray diagram to show where the image of the same object would be formed by an eye suffering from the defect known as hypermetropia or "long-sight". Indicate on your diagram how a suitable correcting lens could be used to bring the image in focus on the retina.

• converging correcting lens (accept bi-convex)
• image formed behind retina without correcting lens (labelled I1)
• corrected image formed on retina (labelled I2)
• lines to show paths of corrected rays when converging correcting lens is used

(4 marks)

(c)

A person suffering from hypermetropia has a near point at a distance of 1.25 m from her eyes.

(i) Calculate the power of the correcting lens needed to bring her near point to 0.25 m from her eyes.

u = 0.25m

v = - 1.25 m

P = 1/u + 1/v = 1/0.25 - 1/1.25

P = 3.2 D

(ii) State whether this power is positive or negative.

positive

(3 marks)

(Total 11 marks)