A concave mirror has a focal length of 20cm. Find the position or positions of an object for which the image-size is double of the object-size.
f=−20cm and m=+2,−2
For a mirror-
1v+1u=1f
Multiplying both sides by u, we get-
uv+1=uf
And, magnification=m=−vu
So, −1m+1=uf
⟹u=f(m−1)m
For, m=2, u=−20(2−1)2=−10cm
For m=−2 , u=−20(−2−1)−2=−30cm