The eye is a sense organ containing receptors sensitive to light intensity…

The idea

A sense organ is a group of receptor cells that detect a specific stimulus. The eye is a sense organ because it contains receptors in the retina that respond to light. Rod cells are sensitive to light intensity, working well in dim light but not detecting colour. Cone cells are sensitive to colour and need brighter light. When light hits these receptors, impulses travel along the optic nerve to the brain, which interprets them as vision. This allows the eye to detect changes in light intensity and colour, helping an organism respond to its surroundings.

For example: In dim light rod cells allow shapes to be seen, while in bright light cone cells allow colours to be distinguished.

What earns the marks

• A sense organ contains receptor cells that detect a specific stimulus.
• The eye is a sense organ because the retina contains receptors sensitive to light.
• Rod cells detect light intensity and work in dim light but do not detect colour.
• Cone cells detect colour and require brighter light than rod cells.
• Impulses from retinal receptors travel along the optic nerve to the brain for interpretation.
• Detecting light intensity and colour helps an organism respond to its surroundings.

Common mistakes and how to put them right

• Saying the eye detects light but not colour.

  Put it right: The eye contains receptors sensitive to both light intensity and colour.

• Thinking rod cells detect colour.

  Put it right: Rod cells detect light intensity, while cone cells detect colour.

• Believing the eye itself produces vision.

  Put it right: Receptors detect light and the brain interprets the impulses as vision.

Exam tips

• Define a sense organ in terms of receptor cells and a specific stimulus.
• Name rod cells for light intensity and cone cells for colour, and state one difference in their light requirements.
• Include the optic nerve and brain when describing how light detection becomes vision.