CBSE Class 10 Science 2020 · Paper 31/3/1 · Question 24(b)

The Human Eye and the Colourful World 3 marks Medium

(a) A person is suffering from both myopia and hypermetropia.

(i)What kind of lenses can correct this defect ?
(ii)How are these lenses prepared ? (b) A person needs a lens of power $+ 3$ D for correcting his near vision and $- 3$ D for correcting his distant vision. Calculate the focal lengths of the lenses required to correct these defects.

Answer

Bifocal lenses correct both defects; the focal lengths are $+ 33.3$ cm for $+ 3$ D and $- 33.3$ cm for $- 3$ D.

(a) (i) Bifocal lenses. (ii) A bifocal lens consists of both a concave and a convex lens: its upper portion is a concave lens, which corrects distant vision, and its lower portion is a convex lens, which corrects near vision. (b) Using

f = \frac{1}{P}

: for

P = + 3

f = \frac{1}{3}

= + 33.3

cm (a convex lens, for near vision); for

P = - 3

f = - \frac{1}{3}

= - 33.3

cm (a concave lens, for distant vision).

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CBSE Class 10 Science 2020 · Paper 31/3/1 · Question 24(b)

Answer

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