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For concave mirror, image obtained at d_2 from mirror for object at d_1 . The focal length of mirror will be ......

Answer»

`f=SQRT(d_1d_2)`
`d_1d_2`
`((d_1+d_2))/(2)`
`sqrt(d_1/d_2)`

Solution :In `1/v+1/u=1/f,u=f+d_1` by taking `v=f+d_2`
`f=(UV)/(u+v)`
`f=((f+d_1)(f+d_2))/(f+d_1+f+d_2)`
`THEREFORE f^2+fd_1+f^2+fd_2=f^2+fd_2+fd_1+d_1d_2`
`therefore f^2=d_1d_2`
`therefore f=sqrt(d_1d_2)`


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