Question

In a concave mirror experiment, an object is placed at a distance $ x_{1} $ from the focus and the image is formed at a distance $ x_{2} $ from the focus. The focal length of the mirror would be

• A. $x_{1} \,x_{2}$
• B. $\sqrt{x_{1}\,x_{2}}$
• C. $\sqrt{x_{1} / x_{2}}$
• D. $\frac{x_{1}+x_{2}}{2}$

Answer 1

Answer: (B)

According to Newton's formula
$f_{1} \, f_{2}=x_{1} \,x_{2}$.
But $f_{1}=f_{2}=f$
$\therefore f^{2}=x_{1} \,x_{2}$
or $f=\sqrt{x_{1}\, x_{2}}$