1. Explain the following terms used in relation to defects in vision and corrections provided for them: (i) Myopia (ii) Astigmatism (iii) Bifocal lenses (iv) Far sightedness(b) Describe with a ray diagram how a person with myopia can be helped by spectacles.OR(a)What is a 'simple microscope? Draw diagrams to show the image formed by a simple microscope with the eye focussed:(i) on near point (ii) at infinity(b)What is the maximum magnication obtainable by a simple microscope?
Answers
Myopia
Near-sightedness tends to run in families.
Faraway objects appear blurry. The condition may develop gradually or rapidly.
Treatment options include glasses, contact lenses and surgery such as LASIK.
Answer:
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Explanation:
(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.