The Human Eye:
The vertical section of human eye is given below. As you can see, eye has the necessary parts:
i) Cornea is transparent part of the eye. Light that enters to eye passes through it. It acts as the protective covering to parts like pupil, crystalline lens etc. and also partly focuses light entering eye.
ii) Iris which serves as muscular diaphragm of variable size which controls size of pupil. Its function is to control amount of light entering to eye. In low light conditions, it dilates pupil and on the other hand, it contracts pupil in high light conditions.
iii) Pupil is the circular aperture in iris.
iv) Eye lens that is supported by the ciliary's muscles and its function is to focus light entering eye onto retina. Action of ciliary's muscles modifies focal length of lens by changing its shape.
v) Retina is light sensitive portion at back inside surface of eye. Optic nerves of brain start at retina from which they transmit massages to brain. Most sensitive spot of the retina is called as yellow spot and its least sensitive portion is blind spot that is where optic nerve leaves eye for the brain. Retina in eye works in same way as the film in the camera. It is interesting to note that brains interpret object scene as right side up.
vi) Cornea is a curved membrane forming front surface of the eye.
vii) Aqueous humor is the transparent liquid between lens and cornea.
viii) Vitreous humor is the jelly liquid between lens and the rest of eye ball.
The optical system of the eye comprises of cornea, the aqueous and vitreous humor and the lens. Rod and cones called as receptors, when stimulated by light, send signals to brain through optic nerves and where the image is perceived. They form the ideal and inverted image of the external object on retina. Retina transmits impression created on it by this image through optic nerve to the brain. Brain then interprets inverted image as being vertical in reality.
Focal length of eye lens is not constant. Shape of lens is altered by action of ciliary muscles to get a convex lens of suitable focal length needed to focus object viewed (far or near) on retina. Ability of lens to focus on near and far objects is called as accommodation.
Power of the Lens:
Power of a lens is stated as the reciprocal of focal length. Where P is power of the lens and f is the focal length. Power of the lens is estimated in diopter (D). For instance when the focal length is 1m, power of the lens is 1D.
P = 1/f
Therefore power of the lens in diopters is provided by expression P = 100(f(cm))
Here focal length is taken in centimeters.
Power of the converging lens is positive while that of the diverging lens is negative as their focal lengths are positive and negative respectively,
Eye Defects and their Corrections:
Closest distance a normal eye can see the object clearly (without accommodation) is known as the near point or least distant of distinct vision. Near point is the closest distance for which lens can accommodate to focus light on retina. This distance is equal to 25 cm for the normal eye. This distance increases with age. It is mentioned in literature that it is approx 50 cm at age 40 and to 500 cm or greater at age 60.
Farthest distance the normal eye can see an object is known as far point and is at infinity for normal eye. Thus, a person with normal eye can see very distant objects like moon.
i) Farsightedness (hyperopia):
In farsightedness (or hyperopia), the person can generally see far away objects clearly but not nearby objects. Light rays don't converged by eye on retina but focuses behind retina. Therefore, image formed by lens in the eye fall behind the retina.
Correction: To correct the defect, the convex lens requires to be placed before eye. It aid in converging furthermore incoming rays before they enter the eye, so that by the time lens in eye converges them, they would accurately fall on retina.
ii) Nearsightedness (or myopia):
When the person can't see clearly or focus to retina objects at far point but can focus on nearby objects, then person is said to be suffering from nearsightedness (or myopia). Generally this problem arises with people who do a lot of reading shows that for nearsighted person, rays from the distance objects get focused before getting to retina.
Correction: The kind of defect can be corrected by using the concave lens placed before eye. It can be seen in the concave lens diverge the rays from distant object before getting to the cornea and thereby enabling the natural lens of the eye to focus the rays on the retina.
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