The eye
Sense organs and the eye
- Sense organs are groups of receptor cells for one kind of stimulus.
- The eye responds to light.
- Light is bent, focused and turned into nerve impulses.
Parts of the eye
| Part | Function |
|---|---|
| cornea | refracts (bends) light as it enters |
| iris | controls how much light enters the pupil |
| lens | focuses the light onto the retina |
| retina | holds the light receptors |
| optic nerve | carries impulses to the brain |
| blind spot | where the optic nerve leaves — no receptors |
Practice
The part of the eye that contains the light receptors is the:
The retina holds the light-sensitive receptor cells; the optic nerve carries their impulses to the brain.
Practice
Which two parts focus light onto the retina?
The cornea bends light as it enters and the lens fine-focuses it onto the retina.
The pupil reflex
- Bright light → the pupil gets smaller, protecting the retina.
- Dim light → the pupil gets wider, letting more light in.
- (Supplement) Done by antagonistic iris muscles: circular muscles narrow the pupil (bright light), radial muscles widen it (dim light).
Practice
In bright light the pupil:
Bright light makes the pupil narrow to protect the retina; dim light makes it widen.
You've got it
Key idea
- light path: cornea (bends) → pupil/iris → lens (focuses) → retina (receptors) → optic nerve → brain
- pupil reflex: bright light → small pupil; dim light → wide pupil
- (Supplement) circular vs radial iris muscles are antagonistic