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.

# 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.

1. A
-6cm
2. B
+6cm
3. C
-12cm
4. D
+12cm

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### 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.

Utilising the mirror formula
$1/v+1/u=1/f$
So now change the settings.
$1/-20+1/-30=1/f$

Hence, option 3 is the correct answer.

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