91Ó°ÊÓ

Object distance p =30cm.

Lateral magnification m=0.500

The image is inverted.

Use the equation of magnification to calculate the image distance. Then we can use the mirror equation to find the focal length of the mirror.

Consider the formula for the magnification as:

$\frac{1}{p}+\frac{1}{i}=\frac{1}{f}$

The mirror equation relates an object distance p, mirror’s focal length f and the image distance i as follows:

$\frac{1}{p}+\frac{1}{i}=\frac{1}{f}$ (1)

The lateral magnification is

$m=-\frac{i}{p}$

But we have the image as inverted, and the absolute value of magnification is 0.500.

So, m= -0.500.

The minus sign shows that the image is inverted.

$$\begin{gathered} -\frac{1}{2}=-\frac{i}{p} \\ i=\frac{p}{2} \end{gathered}$$

Substituting i in (1), and solve as:

$$\begin{gathered} \frac{1}{p}+\frac{2}{p}=\frac{1}{f} \\ \frac{3}{p}=\frac{1}{f} \\ f=\frac{p}{3} \\ =\frac{30.0cm}{3} \\ =10.0cm \end{gathered}$$

Focal length of the mirror is 10.0cm