REVISION MATERIALS FOR CLASS X PHYSICS FOR CBSE

THE HUMAN EYE AND THE COLOURFUL WORLD

FOREVIEW:
This chapter deals with the functioning of human eye, its possible defects and also its remedy. Secondly we shall also discuss several optical phenomena in nature (i.e) about rainbow formation, splitting of white light and also the blue colour of sky.

IMPORTANT CONCEPTS:

  • Human eye can be treated as an optical instrument. Light rays coming from the object to be seen enter the eye through Cornea and fall on the eye lens through the pupil of the eye.
  •  Eye lens being double convex lens forms a real inverted and smaller image of the object on the retina.
  • The retina contains numerous light sensitive cells which are activated by the light falling on the retina and generate electrical signals which are sent to brain via optic nerve and the brain processes this information and we perceive objects as they are.
  • Persistence of vision: The image of an object seen persists on the retina for 1/16 second even after the removal of the object. This continuance of sensation of eye for some timed is called persistence of vision.
  •  Colour blindness: It is said to occur when a person cannot distinguish between colours though his vision may otherwise be normal.
  •  Power of accommodation: It is the ability of the eye to observe distinctly the object situated at widely different distances from the eye on account of change in focal length of eye.
  •  Far Point of the Eye: It is the farthest point at which the object can be seen clearly. For a normal eye, the far point lies at infinity.
  • Near point of the Eye: It is the closest point at which an object can be seen clearly. For normal eye, the near point lies at 25 cm from the eye(least distance of distinct vision)
  • Defects of vision: Following are the common defects of vision (or) refractive defects of eye:

o Myopia(or) Short sightedness: It is that defect of human eye by virtue of which it can see clearly the objects lying at short distances from it. But the far off objects cannot be seen by the Myopic Eye.

Possible causes of Myopia:
1. Increase in the length of the eye ball as if distance of the retina from the eye has increased.
2. Decrease in focal length of eye lens when the eye is fully relaxed.
Remedy: To correct a myopic eye, the person has to wear spectacle with a concave lens of suitable focal length. (i.e.) the focal length of concave lens is equal to the distance of the far point of the myopic eye.

Qn: The far point of a myopic eye is at 50 cm. Calculate the power of the lens to correct the vision.
Ans: The focal length of the concave lens to be used f=-50cm. Therefore P=1/f (metre) = 1/-0.5
= -2D

o Hypermetropia (or) Long sightedness: It is that defect of a human eye by virtue of which it can see clearly the objects lying at large distances from it but the nearby objects cannot be seen clearly.
Possible causes of Hypermetropia: (1) Decrease in length of eye ball as if distance of retina from the eye lens has decreased (2) Increase in the focal length of the eye lens when the eye is fully relaxed.
Remedy: To correct a hypermetropic eye, the person has to wear spectacle with a convex lens of suitable focal length. The focal length of the convex lens is given by f=(x d) / (x-d); where x is the distance of near point of defective eye; d is the distance of near point of normal eye
(25 cm)