A screen is placed 90cm from an object. The image of the object on the screen is formed by a convex lens at two different positions separated by 20 cm. Calculate the focal length of the lens.

 

 

Answers (1)
S safeer

Given: |v| + |u|= 90cm  .....(i)

            |v| - |u| = 20cm ......(ii)

From equation (ii) , we get,

|u| = 35 cm

|v |= 55 cm

we know that,

\frac{1}{f}=\frac{1}{u}+\frac{1}{v}

\frac{1}{f}=\frac{1}{35}+\frac{1}{55}

\frac{1}{f}=\frac{55+35}{55 \times 35}

f=\frac{55 \times 35}{55+35}

f= 21.4cm

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