CBSE Class 10 Science 2020 · Paper 31/3/1 · Question 30(b)

Light - Reflection and Refraction 5 marks Medium

Rishi went to a palmist to show his palm. The palmist used a special lens for this purpose.

(i)State the nature of the lens and reason for its use.
(ii)Where should the palmist place/hold the lens so as to have a real and magnified image of an object ?
(iii)If the focal length of this lens is 10 cm and the lens is held at a distance of 5 cm from the palm, use lens formula to find the position and size of the image.

Answer

Convex lens; palm between $F_{1}$ and $2 F_{1}$ ; image at $v = - 10$ cm, virtual, erect and twice the size ( $m = + 2$ ).

(i) A convex lens (a magnifying glass). With the palm held within its focal length, it forms a virtual, erect and magnified image, so the lines on the palm look bigger and are seen clearly. (ii) The palm should lie between

F_{1}

and

2 F_{1}

of the lens, that is, the lens should be held at a distance between

f

and

2 f

from the palm (between

10

cm and

20

cm for this lens); the image is then real, inverted and enlarged, formed beyond

2 F_{2}

. Holding the palm at

F_{1}

gives a real, highly enlarged image at infinity, which is also accepted. (iii) With

f = + 10

cm and

u = - 5

cm,

\frac{1}{v} = \frac{1}{f} + \frac{1}{u} = \frac{1}{10} - \frac{1}{5} = - \frac{1}{10}

, so

v = - 10

cm: the image is

10

cm from the lens, on the same side as the palm.

m = \frac{v}{u} = \frac{- 10}{- 5} = + 2

: the image is virtual, erect and twice the size of the object.

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CBSE Class 10 Science 2020 · Paper 31/3/1 · Question 30(b)

Answer

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