# Solutions: Medical Option - the EYE

Q11.

(a) A detailed, coloured object is illuminated by white light. Compare what is seen under high intensity light with that seen under low intensity light by an observer with normal eyesight.

(Both answer points, for bright light and dim light, are required to gain each mark)

In bright light cones are the main photodetectors, whereas in dim light only rods are activated.
In bright light colour vision is achieved whereas in dim light only black and white vision is possible.
In bright light detail is clearly seen whereas there is a lack of detail in dim light.
In bright light the image is concentrated around the optic axis but in dim light more image is at the periphery.

(3 marks MAX)

(b) A person suffering from a vision defect has an unaided far point of 2.0 m.

(i) Name this defect of vision.

Short sight or myopia

(ii) Calculate the power of the correcting lens needed to allow the person to see distant objects clearly.

The person needs to view the image of an object at 2.0 m when the actual object is at infinity. The image needs to be on the same side of the refracting system as the object and be upright - it is therefore virtual.

u = infinity

v = -2.0 m

f = ?

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

= 0 - 1/2.0 = -0.5 D

(iii) The person has an unaided near point at 0.22 m. Calculate the aided near point of the person when using the correcting lens.

The person needs to view the image of an object at 0.22 m when the actual object is at the closest point s/he can view it with glasses on. The image needs to be on the same side of the refracting system as the object and be upright - it is therefore virtual.

u = ?

v = -0.22 m

P = -0.5 D

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

- 0.5 = 1/u - 1/0.22

1/u = 1/0.22 - 0.5 = 4.05

u = 0.25 m

(4 marks)

(Total 7 marks)