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Q.

When an object is placed in front of a concave mirror at a distance 30cm from an image is obtained on a screen at a distance of 20 cm from the mirror. Find the focal length of the mirror.


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a

-6cm

b

+6cm

c

-12cm

d

+12cm 

answer is C.

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Detailed Solution

Concept: A convex mirror has a negative focal length, while a concave mirror has a positive one. A concave mirror is a spherical mirror that satisfies the rules of light reflection and has an internally curved reflective surface. It is possible to imagine that the light beams coming from a distant object are parallel to one another.
u=- 30cm
f =- 20cm
Utilising the mirror formula
1/v+1/u=1/f
So now change the settings.
1/-20+1/-30=1/f
∴f =- 12cm
Hence, option 3 is the correct answer.
 
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