A number of optical devices and instruments have been designed utilizing reflecting and refracting properties of mirrors, lenses and prisms. Periscope, kaleidoscope, binoculars, telescopes, microscopes are some examples of optical devices and instruments that are in common use.
- Near point of the eye is 25 cm and far point of the eye is infinity.
- The distance between near point and far point of the eyes is called Range of vision.
Defects of vision:
- Myopia or Short sightedness
- Hypermetropia or Long sightedness
Myopia or Short Sightedness:
A person cannot see far off objects clearly.
- Increase in size of the eyeball i.e., the distance of retina from the eye lens increases.
- A decrease in focal length of the eye lens.
Hypermetropia or Long Sightedness:
A person is not able to see nearby objects clearly. The near point shifts away from 25 cm.
- The contraction in the size of eyeball i.e., distance of retina from eye lens has decreased.
- Increase in focal length of the eye lens.
Presbyopia (old sight):
With increasing age, the ciliary muscles get weakened and thus the power of accommodation of eye decreases with age. Such persons can’t see nearby as well as far off objects clearly. So, their specs would contain both concave and convex lens.
An eye which cannot focus on both horizontal and vertical lines simultaneously is suffering from defects called Astigmatism for such an eye horizontal part of the object will not be visible clearly as the vertical part. This arises when cornea of the eye has diff curvature in diff. directions. This can be corrected by using glasses with the cylindrical lens.