Refraction in the Working of the eye

Laws of Refraction

—The incident and refracted rays are on the opposite sides of the normal at the point of incident and all three are in the same plane.

The ratio of the sine of the angle incidence to the sine

of the angle of refraction is a constant for a given pair of media.

i.e. Sin i/Sin r = constant

(The constant is the refractive index of the second medium with respect to the first medium)

The Eye

Important Parts of the Eye and their Functions

—Cornea: The front part of the eye is covered by a transparent spherical membrane called the cornea. Light enters the eye through cornea. The space behind the cornea is filled with a liquid called aqueous humor .

—Iris: Just behind the cornea is a dark coloured muscular diaphragm which has a small circular opening in the middle.

Pupil: Pupil is the small circular opening of iris. The pupil appears black because no light is reflected from it.

*The iris regulates the amount of light entering the eye by adjusting the size of the pupil

How iris regulates the amount of light entering the eye?

When the intensity of light is more or if it is a bright source of light then the iris makes the pupil to contract and as a result the amount of light entering the eye decreases

When the intensity of light is less or if the light is dim then the iris dilates the pupil so that more light can enter the eye

—Eye Lens: The eye lens is a convex lens made of a transparent jelly - like proteinaceous material. The eye lens is hard at the middle and gradually becomes soft towards the outer edges. The eye lens is held in position by ciliary muscles. The ciliary muscles help in changing the curvature and focal length of the eye - lens.

—Blind Spot: It is a spot at which the optic nerve enters the eye and is insensitive to light and hence the name.

Retina: The inner back surface of the eye ball is called retina. It is a semi-transparent membrane which is light sensitive and is equivalent to the screen of a camera. The light sensitive receptors of the retina are called rods and cones. When light falls on these receptors they send electrical signals to the brain through the optic nerve. The space between the retina and eye lens is filled with another fluid called vitreous humor.

Working on an eye

The light coming from an object enters the eye through cornea and pupil.

The eye lens converges these light rays to form a real, inverted and diminished image on the retina.

—The light sensitive cells of the retina gets activated with the incidence of light and generate electric signals.

These electric signals are sent to the brain by the optic nerves and the brain interprets the electrical signals in such away that we see an image which is erect and of the same size as the object.