CBSE Class 10 Science 2019 · Paper 31/5/1 · Question 7(b)

Light - Reflection and Refraction 3 marks Medium

A real image

2/ 3^{rd}

of the size of an object is formed by a convex lens when the object is at a distance of 12 cm from it. Find the focal length of the lens.

Answer

The focal length is $f = + 4.8$ cm (image distance $v = + 8$ cm).

The image is real, so it is inverted:

m = - \frac{2}{3}

. With

u = - 12

cm,

v = m \times u = + 8

cm, and

\frac{1}{f} = \frac{1}{v} - \frac{1}{u} = \frac{1}{8} + \frac{1}{12} = \frac{5}{24}

, so the focal length of the lens is

f = \frac{24}{5} = + 4.8

cm.

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