Question

Identify the wrong sign convection:
A) The magnification for a virtual image formed by a convex lens is positive.
B) The magnification for real images formed by a convex lens is negative.
C) The height measured normal to the principal axis upwards is positive.
D) The distance measured in the direction of incident light is positive.
E) The magnification for a virtual image formed by a concave lens is negative.

Answer 1

The sign conventions of the lens and mirror are important and must be remembered and followed the way they are made. The sign conventions for lens and mirror are the same but the formula may differ from each other.

Formula used:
The formula of the magnification is given by,
$\Rightarrow m = \dfrac{v}{u}$
Where the magnification is m, the image distance is v and the object distance is u.

Complete step by step solution:
It is asked in the problem about the sign convention of the magnification of real and virtual images, here we need to find the wrong option. We will consider all the options and find which option is not correct.
Let us consider the option A.
The magnification for a virtual image formed by a convex lens is positive. The formula of the magnification is equal to,
$\Rightarrow m = \dfrac{v}{u}$
Where magnification is m and the image distance is v and the object distance is u.
The object distance and image distance both will be equal to negative and therefore the magnification will be positive, so option A is the correct statement.
Consider option B.
The magnification for real images formed by a convex lens is negative. The object distance and the image distance are negative and positive and therefore the magnification is negative because magnification is given by,
$\Rightarrow m = \dfrac{v}{u}$
Let us consider option C.
The height measured normal to the principal axis upwards is positive. The height measured normal to the height of the principal axis is upwards is taken positive and if the image height is measured in downwards direction then its height is taken as negative. So this option is also correct about measurement of height.
Consider option D.
The distance measured in the direction of incident light is positive.
The distance which is measured in the direction of incident light is always taken as positive and the distance measured in the direction opposite to the direction of the incident ray is taken negative. So this option is correct.
Consider option E.
The magnification for a virtual image formed by a concave lens is negative.
The magnification of the concave lens is positive because the object and the image both are formed on the left side of the lens and also the height is upwards the principal axis. So this option is not correct.

The wrong sign convection is option E.

Note: The students are advised to understand and remember the formula of magnification for lens and mirror as it is very helpful in solving the problem like these. The magnification of a mirror has an extra negative sign and the magnification of the lens.