2.15B Explain the structure and function of the eye as a sensory receptor…

The idea

The eye is a sensory receptor that detects light and sends impulses along the optic nerve to the brain. Light first passes through the cornea, a transparent curved layer that refracts (bends) light entering the eye. The lens then refracts light further to focus it onto the retina; the lens changes shape to focus on near or distant objects. The iris is the coloured muscle that controls the size of the pupil, widening it in dim light and narrowing it in bright light to control how much light reaches the retina. The retina contains rod cells, which work in dim light and detect black and white, and cone cells, which work in bright light and detect colour. Together these cells convert light into nerve impulses.

What earns the marks

• States that the cornea is transparent and refracts light entering the eye.
• States that the lens refracts light to focus it on the retina and can change shape to focus near or distant objects.
• States that the iris controls the size of the pupil to regulate how much light enters the eye.
• States that rod cells in the retina detect dim light and black-and-white images.
• States that cone cells in the retina detect bright light and colour.
• Links the retina to converting light into nerve impulses sent to the brain.
• Recognises the eye as a sensory receptor that detects light stimuli.

Common mistakes and how to put them right

• Saying the cornea focuses a sharp image on its own.

  Put it right by explaining the cornea refracts light and the lens does the fine focusing.

• Confusing the iris with the pupil.

  Put it right by stating the iris is the muscle that controls the pupil's size.

• Swapping rod and cone functions.

  Put it right by linking rods to dim light and black-and-white, cones to bright light and colour.

• Treating the eye as producing light rather than detecting it.

  Put it right by describing the eye as a receptor for light.

Exam tips

• Match each named part to one clear function and learn them as pairs.
• Use the words refract and focus precisely when describing the cornea and lens.
• For rod and cone questions, always state the light level and the type of image detected.