(a) What is myopia? State the two causes of myopia. With the help of labelled ray diagram, show
(i) The eye defect myopia
(ii) Correction of myopia using a lens. (b) Why is the normal eye unable to focus on an object placed within 10 cm from the eye?
Answers
Answer:
a short sightedness or near sightedness is referred as myopia.
causes:
- due to the high converging power of the eye lens .
- due to the eye ball being too long .
Answer:
(a) (i) Myopia: Short-sightedness is caused due to excessive curvature in cornea or elongation of eyeball. Image is formed before of the retina. So, a concave lens is used to correct it.
(ii) Astigmatism: The inability to focus the light in both vertical and horizontal lines is called astigmatism. It is caused due to varying curvature in lens both horizontally and vertically. It is . corrected by using cylindrical lens.
(iii) Bifocal lenses: These are used to correct presbyopic eye. These contain lenses with upper concave and lower convex surface. Presbyopia arises with age.
(iv) Far-sightedness: Hypermetropia or far-sightedness is caused due to greater focal length of eye lens and/or when eyeball becomes smaller. Image is formed beyond the retina and can be corrected using a convex lens.
(b) The focal length of the lens cannot be changed up to an extent that objects nearer than 25 cm can be viewed (its near point is 25 cm). So it is not able to focus for 10 cm.
Explanation: