Question

# In a concave mirror an object is placed at a distance x from the focus, and the image is formed at a distance y from the focus. The focal length of the mirror is

A
xy
B
xy(xy)
C
x+y2
D
xy

Solution

## The correct option is B $$\dfrac{x y}{(x-y)}$$$$\begin{array}{l} u=-x \\ v=-y \\ f=+f \\ \dfrac{1}{v}-\dfrac{1}{u}=\dfrac{1}{f} \\ \frac{1}{-y}+\dfrac{1}{x}=\dfrac{1}{+f} \\ \dfrac{x-y}{x y}=\dfrac{1}{f} \\ \dfrac{x y}{(x-y)}=f \end{array}$$Physics

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