Page 2 - Lesson note - introduction

7. Pupil: It regulates and control the amount of light entering the eye. It is the black
opening between aqueous humour&lens. Black in colour. Light entering cannot exit.
Under the muscular action of the iris, the size of the pupil becomes smaller in bright
light and larger in dim light.

8. Crystalline eye lens: Provides the focused real & inverted image of the object on
the retina. It is composed of a fibrous, jelly like material.
This is convex lens that converges light at retina.

9. Ciliary muscles: It helps to change the curvature of eye lens and hence changes
its focal length so that we can see the object clearly placed at different positions.

10.Retina; It is a delicate membrane of nerve fibers on the inner side of the backwall
of the eye. It contains light sensitive cells called rods and cones. Rods are sensitive
to intensity of light while cones are sensitive to colors. These cells change light
energy into electrical signals which send message to the brain via the optic nerves.
11. Blind spot and yellow spot. In the region where the optic nerve enters the
eyeball, there are no rods and cones. This region is totally insensitive to light and is
called blind spot. Yellow spot has maximum concentration of light sensitive cells. It is
situated in the center of the retina.

12. Aqueous humour and vitreous humour.; Aqueous humour is a salty fluid (μ =
1.337) that fills the space between the cornea and the eye lens. Vitreous humour is a
jelly like fluid (μ = 1.437) that fills the space between the retina and the eye lens.

How Pupil Works?

Example: You would have observed that when you come out of the cinema hall after
watching movie, in the bright sun light, your eyes get closed. And when you enter the
hall from the bright light, you won ‘be able to see but after some time you would be
able to see.

Here the pupil of an eye provides a variable aperture, whose size is controlled by iris
(a) When the light is bright: Iris contracts the pupil, so that less light enters the eye.
(b) When the light is dim: Iris expand the pupil, so that more light enters the eye.
Pupil opens completely, when iris is relaxed.

Action of the eye
The transparent structures like cornea, aqueous humour, eye lens and vitreous
humours together constitute a single converging lens. As the rays from an object
enter the eye, they suffer refractions on passing successively through these
structures and get converged. A real and inverted image is formed on the retina. The
light sensitive cells of retina get activated and generate electrical signals that are
sent to the brain through the optic nerves. Our brain translates the inverted image
into an erect image.

Some important terms:

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