Question

A man uses a concave mirror for shaving . he keeps his face at at a distance of 25cm from the mirror and gets an image which is 1.4 times enlarged. find the focal length of the mirror
​

Answer 1

Answer:

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Explanation:

u=−25cm

m=

AB

A

′

B

′

​

=−

u

v

​

⇒1.4=−(

−25

v

​

)⇒

10

14

​

=

25

v

​

'

⇒v=

10

25×14

​

=35cm

Now,  

v

1

​

+

u

1

​

=

f

1

​

⇒

f

1

​

=

35

1

​

−(

−25

1

​

)=

175

5−7

​

=−

175

2

​

⇒f=−87.5cm

So, focal length of the concave mirror is 87.5cm.

Answer 2

Explanation:

f=-87.5 cm

therefore focal length of concave mirror is 87.5 cm