ACCOMMODATION OF AN EYE

 ACCOMMODATION OF AN EYE 

      A normal eye can see the near and far off objects clearly if the sharp images of these objects are formed on the retina. Since the distance between the eye lens and retina is fixed, so to see the objects at different positions from the lens, the focal length of the lens has to be changed accordingly to form the sharp images of these objects.

                                                       If the objects to be seen is far off, then the sharp image of this object can be formed on the retina of the eye by increasing the focal length of the eye lens. The focal length of a lens increases if its thickness decreases. Thus, to decrease the thickness of the eye lens and hence to increase its focal length, ciliary muscles are completely relaxed. Now the parallel beam of light coming from the far off objects is focussed on the retina and hence the object is seen clearly.

                                          

If the object to be seen in close to the eye i.e., near to the eye, then the sharp image of this object can be formed on the retina of the eye by decreasing the focal length of the eye lens. The focal length of a lens decreases if its thickness increases. Thus, to increase the thickness of the lens and hence to decrease its focal length, ciliary muscles contract. Now the beam of light coming from the near object is focussed by the lens on the retina. Hence the object is seen clearly.

Conclusion : Thus, we observe that the focal length of the eye lens is adjusted automatically by the action of ciliary muscles such that a sharp images of the objects at different positions from the eye are formed on the retina. This process is known as accommodation of an eye. 

         

           Definition of Accommodation : The ability of an eye to change the focal length of its lens in such a way that the sharp image of an object can be placed so that its sharp image is formed on the retina is known as power of accommodation of the eye. 

           Near Point : The nearest point from an eye at which an object can be placed so that its sharp image is formed on the retina without strain is known as the near point of the eye.

           Far Point : The farthest position of an object from a human ye so that the sharp image of the object is formed on the retina of the eye is known as far point. For a normal eye, far point is at infinity.

           Range of vision : The distance between the near point and far point of an eye is known as range of vision.

      

             For a normal eye, the range of vision is 25 cm to infinity. 

  (Do You Know?) : -

Focal length of an eye lens cannot be decreased below a certain limit.

Least distance of distinct vision or near point of a normal eye of an adult is 25 cm.