Question

A student holding a mirror in his hand directed the reflecting surface of the mirror towards the sun. He then directed the reflected light on a sheet of paper held close to the mirror.
(a) What should he do to burn the paper?
(b) Which type of mirror does he have?
(c)Will he be able to determine the approximate value of focal length of this mirror from this activity? Give reason and ray diagram.

Answer 1

Mirror which has the converging properties will be used to burn the paper because to burn the paper we need to converge the sunlight’s intensity at one point. That kind of mirror is a concave mirror.
We will place the paper at such a point where the intensity of light will be maximum, that is the principal focus of the mirror.

Complete step by step solution:
Let’s answer each of the above questions one by one:

-To burn the paper distance between the paper and mirror must be equal to the focal length of the mirror because using this distance sun rays get converged to the principal focus of the mirror F.
-Students have a converging type of mirror called concave mirror .Concave mirrors have the property that when the object is placed at infinity the image formation takes place at principal focus of the mirror. The image formed is highly diminished (much smaller than the object).
-Focal length is the distance between the pole of the mirror and principal focus of the mirror. Yes we can easily find out the focal length of the mirror by knowing the distance of the object and distance of the image formed from the pole of the mirror. We can use the formula : $\dfrac{1}{f} = \dfrac{1}{u} + \dfrac{1}{v}$ ( f is the focal length here, u is the distance of the object from pole P and v is the distance of the image formed ).

Note: Concave mirrors due to the converging properties are used as shaving mirrors, head mirrors used by doctors especially dentists, used in ophthalmoscope, astronomical telescope, head lights of the vehicles, solar furnaces because concave mirror can focus sun rays at particular point .