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The diagram below shows how the iris and pupil of a human eye appear under different conditions.

(a) Name the structures labeled X and Y
(b) (i) State the condition that leads to the change
in appearance shown in the diagram labeled B
(ii) Describe the changes that lead to the appearance of the iris and pupil as shown in the diagram labeled B
(iii) What is the significance of the changes described in (b) (ii) above?

Answer Text:
(a) - X: pupil
- Y: Circular muscle
(b) (i) Dim light/ low light intensity/ darkness
(ii) - Circular muscles in iris relax, while radial muscles contract. The pupil becomes bigger allowing more light to enter the eye.
(iii) - Allows one to visualize or see objects under
dim light.