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Q.

The image of an object formed by a mirror is real, inverted and the magnification is -1. When, the image is at a distance of 40 cm from the mirror, if the object is moved 20 cm towards the mirror image would be formed:


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a

Beyond C

b

Between C and f

c

At infinity

d

Behind the mirror 

answer is C.

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Detailed Solution

The image of an object formed by a mirror is real, inverted and is of magnification -1. The image is at a distance of 40 cm from the mirror, if the object is moved 20 cm towards the mirror image would be at infinity.
It has been given that,
Magnification, m=-1
Image distance, v=-40 cm
Now, m=-vu
-1= --40u
u=-40 cm
So, object and image both are in the same position. Therefore this point must be the center of curvature.
Now, since the magnitude of f is half of the center of curvature,
f=402
f=20 cm            Now, if the object is moved 20 cm towards the mirror then its new position would be at the focus of the mirror.
Object distance, (u)=-20 cm
Focal length (f)=-20 cm
We know that when an object is kept at the focus of a concave mirror, the image is formed at infinity.
 
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