'A' Level Medical Option Questions - The EYE
Q12.A person suffering from long sight has an unaided near point 63 cm from the eye.
Above are three diagrams (not drawn to scale).
The first two diagrams show rays incident on the unaided eye.
The third diagram shows rays incident on the correcting lens which will allow the person to have an aided near point 25 cm from the eye.
(a) Complete the diagrams to show the passage of the rays to the retina. You may assume that for the eye there is only a single refraction at the cornea of the eye.
first two diagrams correct 
third diagram correct
(2 marks)
(b) Calculate the focal length of the correcting lens in metres, stating the answer to the appropriate number of significant figures.
u = 0.25 m
v = -0.63 m
1/f = 1/u + 1/v
1/f = 1/0.25 - 1/0.63 = 2.41
f = 0.41 m(correct 2sf )
(3 marks)
(c) Explain what is meant by persistence of vision and state a practical situation where it is important.
Persistance of vision is where we experience the image remaining after the stimulus has been removed. This happens for a very short period of time. If a set of still pictures is flashed at us fast enough we are not aware that they are single individual images. This is because the first image is still being perceievd when the second one appears in the field of vision. There therefore does not appear to be a momentary gap between them. eg cinema pictures, television, fluorescent lights, optical illusions
(2 marks)
(Total 7 marks)
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