Question

If the object and image distances from the focus of a concave mirror are $x$ and $y$, then the focal length of the mirror is

• A. $\frac{2xy}{x+y}$
• B. $\frac{x+y}{2}$
• C. $\frac{xy}{x+y}$
• D. $\sqrt{xy}$

Answer 1

The correct option is D $\sqrt{xy}$

According to Newton 's mirror relation,

cc' = f${}^2$ where c is object distance and c' is image distance, both from focus

Thus as per given question, f=$\sqrt{xy}$