question_answer

In a concave mirror, an object is placed a distance \[{{d}_{1}}\]from the focus and the real image is formed at a distance dg from the focus. Then the focal length of the mirror is                      

A)  \[\sqrt{{{d}_{1}}{{d}_{2}}}\]        

B)         \[{{d}_{1}}{{d}_{2}}\]

C)  \[\frac{{{d}_{1}}{{d}_{2}}}{2}\] 

D)         \[\sqrt{{{d}_{1}}/{{d}_{2}}}\]