real and inverted image to the brain through optical nerves. Finally, the brain senses it as erect image. Power of Accommodation The ability of the eye lens to focus nearby as well as the distant objects is called power of accommodation of the eye. This is achieved by changing the focal length of the eye lens with the help of ciliary muscles.
Eye lens is made of a flexible, jelly-like material. By relaxing and contracting the ciliary muscle, the curvature and hence the focal length of the eye lens can be altered. When we see distant objects, the ciliary muscle relaxes and makes the eye lens thinner. This increases the focal length of the eye lens.
Hence, the distant object can be clearly seen. On the other hand, when we look at a closer object, the focal length of the eye lens is decreased by the contraction of ciliary muscle. Thus, the image of the closer object is clearly formed on the retina. Persistence of vision If the time interval between two consecutive light pulses is less than second, human eye cannot distinguish them separately.
It is called persistence of vision. The far point and near point of the human eye The minimum distance required to see the objects distinctly without strain is called least distance of distinct vision. It is called as near point of eye. It is cm for normal human eye.
Optics The maximum distance up to which the eye can see objects clearly is called as far point of the eye. It is infinity for normal eye. . DEFECTS IN EYE A normal human eye can clearly see all the objects placed between 25cm and infinity.
But, for some people, the eye loses its power of accommodation. This could happen due to many reasons including ageing. Hence, their vision becomes defective. Let us discuss some of the common defects of human eye.
Myopia Myopia, also known as short sightedness, occurs due to the lengthening of eye ball. With this defect, nearby objects can be seen clearly but