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  • concave mirror forms a real image of an object kept at a distance 9 cm from it. If the object is taken away from the mirror by 6 cm, the image size reduces to <img alt="\frac{1}{4}^{th}" src="

concave mirror forms a real image of an object kept at a distance 9 cm from it. If the object is taken away from the mirror by 6 cm, the image size reduces to \frac{1}{4}^{th} of its previous size. Find the focal length of the mirror.

 

Answers (1)

best_answer

As we already know,  

m = \frac{f}{f-u}

In the first case, u = 9cm

m1 = \frac{f}{f+9}

And in the case 2, u = 15cm

m2 = \frac{f}{f+15}

m1 = 4m2

\frac{f}{f+9} = 4 \frac{f}{f+15}

f+15 = 4f+36

3f = -21

f = -7cm

Posted by

Divya Sharma

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