2.5 human eye

what is focusing?

the adjustment of the lens of the eye so that clear images of objects at diff distances are formed on the retina

describe what happens when producing a focused image of a distant object

1. ciliary muscles relax

2. suspensory ligaments become taut

3. edge of lens is pulled and lens becomes thinner and less convex

4. focus on the retina (fovea)

describe what happens when producing a focused image of a near object

1. ciliary muscles contract

2. suspensory ligaments slacken

3. (elastic) lens becomes thicker and more convex

4. focus on the retina (fovea)

define binocular vision

the usage of two eyes to perceive a single 3D image of the surroundings

what is the pupil reflex?

a reflex action which is protective in nature

describe the nervous pathway in the pupil reflex

1. stimulus (change in light intensity)

2. receptors (in retina)

3. sensory neurone in the optic nerve

4. relay neurone in brain

5. motor neurone

6. effector (iris muscles)

describe the muscles in the iris

• circular — arranged in a circle around the pupil

• radial — radiate outwards from the pupil

• they are antagonistic

describe how the iris controls the amount of light entering the eye in bright light

1. circular muscles of the iris contract

2. radial muscles of the iris relax

3. pupil constricts, reducing the amount of light entering the eye

describe how the iris controls the amount of light entering the eye in dim light

1. circular muscles of the iris relax

2. radial muscles of the iris contract

3. pupil dilates, increasing the amount of light entering the eye