Coordination and Response: Question 2
Syllabus 14.2
A planetarium usher spends each show inside the dimly lit projection dome, guiding visitors to their seats by torchlight. Between shows, the usher steps out into the brightly lit foyer to greet the next group of visitors.
(a) State the name of the eye structure that: (i) refracts light as it enters the eye (ii) contains the light receptors (iii) carries impulses from the eye to the brain [3]
(b) Describe how the diameter of the usher's pupils changes as they step from the dim dome into the bright foyer, and explain why this response is useful. [3]
Show worked solution Hide worked solution
Worked solution
Part (a): Naming eye structures and their functions
Each part of the eye has a distinct role in forming an image and sending information to the brain:
- (i) Cornea: the transparent front layer of the eye that refracts (bends) light as it enters.
- (ii) Retina: the layer at the back of the eye that contains the light receptors.
- (iii) Optic nerve: carries electrical impulses from the retina to the brain.
Note that the lens also refracts light, but its specific function is to fine-focus light onto the retina. The cornea does most of the initial refraction as light enters the eye.
Part (b): The pupil reflex
As the usher steps from the dim projection dome into the brightly lit foyer, the light intensity reaching their eyes increases sharply.
In response, the diameter of the pupil decreases. The pupil gets smaller.
This is useful because a smaller pupil lets less light into the eye. Without this response, the sudden bright light in the foyer would let too much light fall on the retina, overstimulating (and potentially damaging) its light receptors. By narrowing the pupil, the eye keeps the amount of light reaching the retina at a level it can process comfortably.
Final answers
- (a) (i) cornea (ii) retina (iii) optic nerve
- (b) The pupil gets smaller, reducing the amount of light reaching the retina and protecting its light receptors from being overstimulated.