To get the best deal on Tutoring, call 1-855-666-7440 (Toll Free)


The eyes of humans and all living creatures are the wonderful examples of optical instruments. An eye basically does the function of seeing an object and nature has equipped it with all facilities for its function. It has an aperture which can flexibly be adjusted depending on the situations, a lens which can adjust its power and a screen called Retina for screening the image of the object. In ideal conditions, the image is focused exactly on the retina. However, for various reasons, the projection of the image on the retina gets disturbed. Let us study how such things happen.


What is Farsightedness?

Back to Top
Some people are able to clearly see objects at far distances but cannot see nearby objects clearly. This defect in their eyes is called far sightedness because the far off subjects are able to be sighted. However, this only a defect developed in the eyes and it is not a disease.
There are many reasons for the cause of this defect. Most of the elderly persons nearing the age of fifty acquire this defect almost as a natural phenomenon. This defect is not an alarming one and can be easily corrected with spectacles fitted with convex lens of required power. The medical term for this defect is ‘Hypermetropia’ or ‘Hyperopia’. Persons having this defect are called ‘Farsighted’ or ‘Hyperopic’, like we call the persons having diabetes as diabetic.

Hyperopia Definition

Back to Top
As we mentioned Hypermetropia, also called Hyperopia, is a defect in an eye due to which the eye can see clearly only the farsighted objects. The nearby objects will not be clear and with this defect will not be able to read letters which are close by. At an advanced level of hypermetropia one may not be able to clearly see the objects even at a few feet away.Let us study how Hyperopic persons are easily treated by use of a convex lens. Look at the following diagram.
Figure (i) shows the eye is not able to focus the rays on the retina due to insufficient refraction. In figure (ii), the use of convex lens to overcome the defect is explained. The rays are converged by the convex lens to the extent required with which the eye lens can correctly focus the rays on the retina. This is because the property of convex lens is that converges the rays fall on that. Let us see how practically is this done and how the power of the convex lens is calculated for correcting the Hypermetropia.

The defect occurs due to change of shape of the eye ball because of various reasons. The focal length of the lens of a normal human eye is about 17 mm. But the change in shape sometimes increases the focal length of the eye lens. This results in formation of the image beyond the retina. Hence a convex lens is required to be placed in front of the eye. The combined focal length of eye lens and the convex lens of correct power equals the required focal length to focus the image on the retina. The approximate formula for combined focal length ‘ f ’ of two lenses side by side is,

$\frac{1}{f}$ = $\frac{1}{f_{1}} + \frac{1}{f_{2}}$

In practice, the power of the lens to correct the hypermetropia is not figured out by this method. The reasons are simple. You do not measure the actual and ideal focal lengths of the eye of a person. Hence what is done is, the person is made to view an object (usually words) at a certain distance. Lenses of different powers are tried one by one and the right power is chosen when the person is able to read the words clearly.


Back to Top
Let us now study what exactly is the reason for people to become farsighted. When an object is close to the eyes, the eye must increase its ability to keep the image in the retina. Also the refractive index of the eye lens should be such that it refracts the light rays to the correct angle so as to the refracted rays focus at the retina. Inability of any of these and or due to other reasons, the rays gets ‘under refracted’ and hence the focusing deems to take place behind retina. As a result the retina does not get a clear image resulting in blurred vision of the object. The ability to see far off objects and the inability to clearly see the nearby objects, provide the symptoms of farsightedness.
A child which can spot a ball at a distance easily but unable to catch that is sure to have Hypermetropia. Because, when the ball gets closer and closer, the vision of the ball gets more and more blurred. Similarly, a child spills food while eating may have developed Hypermetropia. When such symptoms are noticed with the children, the parents must immediately go for a optical check. In many cases, small babies may be having Hypermetropia because of small eyes. But over a period of time, during their growth, the defect gets settled by itself. This is also the reasons why ophthalmologists leave mild Hypermetropia untreated in case of small children. Rather they prefer healthier methods to improve the eye power by administration of necessary vitamins and nutrition supplement medicines.
Related Topics
Physics Help Physics Tutor
*AP and SAT are registered trademarks of the College Board.