Question

A plane mirror is used as a magnifying glass  

a) True

b) False

Answer 1

Hint: Magnification is always defined as the ratio of the  height of the image to that of height of the object. If the value of  magnification produced by an optical system is greater than 1, we can conclude that the height of the image is greater than that of the object, hence we say that the image has been magnified. If in an optical system, the height of the image is equal or less than that of the object then that optical system cannot be used to see the magnifying image of the object. Hence we will see whether a plane mirror produces a magnified image and accordingly draw the conclusion whether it can be used as a magnifying glass. 

Complete answer:
Let us place an object of height h in front of the mirror at a distance U.

From the above diagram we can see that the image is formed at a distance V from the mirror. Let us say the height of the image formed is H. Hence, the magnification produced by the mirror is given by,

$m=\dfrac{H}{h}$ .The magnification of any mirror is also given by, $m=\dfrac{-V}{U}$ where V is the image distance and U is the object distance from the mirror. In a plane mirror the image distance equals to the object distance i.e. V=-U. The negative sign indicates that the image and object are on the either side of the mirror . Hence we can say that the magnification produced by the plane mirror i.e. $m=\dfrac{-V}{U}=\dfrac{-V}{-V}=1$ is unity. Hence from this we can conclude that a plane mirror cannot be used as a magnifying glass.

So, the correct answer is “Option B”.

Note:
A magnifying glass usually consists of convex lenses and not mirrors. This is because it's more convenient when the object is placed within the focal length of the lens, the image formed is on the same side of the lens. Usually concave mirrors are used to obtain the magnified image of the object and not any other mirrors.