Human eye and its resolving power

 HUMAN EYE

The human eye is like a camera in its basic structure  but is more complex. The interior of the eye is filled with a transparent gel-like substance called the vitreous humor with index of refraction. 

n = 1.337. Light enters in this region through the cornea and lens. Between the cornea and lens is a watery fluid,the aqueous humor with n = 1.336. The iris adjusts automatically to control the amount of light entering the eye, similar to a camera.The retina, which plays the Role of the film or sensor in a camera, is on the curved back Lens surface of the eye. The retina is composed of a many nerves which act to change light Cornea energy into electrical signals that travel along the nerves to Aqueous brain for interpretation. 

The lens of the eye (n = 1.386 to 1.406) does little of the bending of the light rays.

Most of the refraction is done at the front surface of the cornea at its interface

with air. The lens acts as a fine adjustment for focusing at different distances. This

is accomplished by the ciliary muscles, which change the curvature of the lens so

that its focal length is changed.To focus on a distant object, the ciliary muscles of the eye are relaxed and the lens is thin, 

 (a) ::: and parallel rays focus at the focal point (on the retina). To focus on a nearby object, the muscles contract, causing the centre

of the lens to thicken, 

(b), ::: thus shortening the focal length so that

images of nearby objects can be focused on the retina, behind the new focal point.

This focusing adjustment is called accommodation.

 

The closest distance at which the eye can focus clearly is called the near point of

the eye. For young adults it is typically 25 cm, as people grow older, the ability to

accommodate is reduced and the near point increases.A given person's far point is the farthest distance at which an object can be seen clearly. Since we can focus on the Moon and stars, it is clear that the normal far

point is essentially infinity.Many people have eyes that do not accommodate within the normal range of 25 cm to infinity or have some other defect. Two common defects are short-sightedness and long-sightedness. Both can be corrected to a large extent with lenses-either eyeglasses or contact lenses.

(a)::Short sightedness 

In short-sightedness (nearsightedness or myopia), the human eye can focus only on nearby objects. The far point is not infinity but some shorter distance, so distant objects are not seen clearly.Short-sightedness is usually caused by an eyeball that is too long, although sometimes it is the curvature of the

cornea that is too great. In either case,

images of distant objects are focused in

front of the retina.. 

(a)::A diverging lens, because it

causes parallel rays to diverge, allows

the rays to be focused at the retina. 

(b) and thus can

correct nearsightedness.

(b):: Long-sightedness:

 Long-sightedness

long-sightedness (farsightedness or

hyperopia), the eye cannot focus on

nearby objects. Although distant

objects are usually seen clearly, the

near point is somewhat greater than

the “normal” 25 cm, 

(a). This defect is caused by an

eyeball that is too short or (less often)

by a cornea that is not sufficiently

curved. It is corrected by a converging 

lens,

Resolving power: 

Resolving power is the capacity of an instrument to distinctively separate two points which are close together. The sharpness of vision-in particular, the ability to visually separate closely spaced objects-is

referred to as resolution.the headlights on a car. As the car approaches closer, the separation between For example, in the first photo we see a bright light in the distance that may be the single headlight of an approaching motorcycle or the unresolved image of two lights will increase. As the car continues to approach, its individual headlights become increasingly distinct, as shown in the third photo. So the two points are distinctively separated.