Question

The radius of curvature of a concave mirror is 30cm. What is the focal length of this mirror.¿?

Answer 1

Given :-

Radius of a concave mirror is 30 cm.

• R = 30 cm

To Find :-

• Focal length of the mirror

Point to remember :-

Focal length:-The distance from the lens to the focal point is called the focal length.

For converging lenses, the focal length is always positive, while diverging lenses always have negative focal lengths.

Formula used :-

$\purple{\mid{\fbox{\rm{R = 2F}}\mid}}$

i.e. Radius is Double of the focal length.

Solution :-

$= > 30 \: cm = 2 \times F \: \: \: \: \: \: \: \: \: \: \: \\ = > 30 \: cm = 2F \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \\ = > F = \frac{30 \: cm}{2} = 15cm$

Therefore,

Focal length of the given mirror with radius of 30 cm is 15 cm.

$\bf\underline \red{\underline{Hope \: it \: helps }}$

Answer 2

Answer:

Explanation:

           

• For spherical mirrors having a small aperture, the radius of curvature is twice the focal length.

• Let R be the radius of curvature and f be the focal length then,

        $R = 2f$     ...(1)

        and  

        $f = \frac{R}{2}$       ...(2)

   

In the question, it is given that,

$R= 30\ cm = 30\times 10^{-2} m$

Substitute  the value of 'f' into equation(1),

We get,

$f =\frac{30\times 10^{-2}}{2} = 15\times10^{-2} m = 15\ cm$

• The focal length will be $15\ cm$