What is hypermetropia? Write two causes for development of this defect. Describe with a ray diagram how this defect of vision can be corrected by using spectacles.
A person can see distant objects distinctly but cannot see nearby objects so clearly in this case, the image is formed behind the retina. This defect of the eye is hypermetropia.
Causes of hypermetropia: This defect arises because either
- the focal length of the eye lens is too long, or
- the eyeball becomes too short, so that light rays from the nearby object, say at point N, cannot be brought to focus on the retina to give a distinct image.
Hypermetropia can be corrected by using convex lens of suitable focal length in spectacles.