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Question

When an object is 40cm away from pole of a concave spherical mirror, the size of the image is equal to that of object.
The focal length of the mirror is__________


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Solution

Step 1: Given data

Given: u is the object distance from pole and v is the image distance
u=-40cm
m is magnification
m=-1
Step 2: Finding the value of v

We have a magnification formula,

m=-vu-1=-vuv=u=-40cm

Step 3: Finding the value of focal length

We have a formula,1u+1v=1f

-140-140=1f

-240=1f

f=-20cm
Thus, the focal length of the mirror is 20cm.


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