There are 2 formula is using to find focal length.

1/f = 1/v + 1/u and the other is

1/f= 1/v - 1/u.

How to select the formula to find focal length according to the problem of grade 10 CBSE student?

There are 2 formula is using to find focal length.

1/f = 1/v + 1/u and the other is

1/f= 1/v - 1/u.

How to select the formula to find focal length according to the problem of grade 10 CBSE student?

1/f = 1/v + 1/u

f= focal length. Know where to use focal length in positive and where in negative. The sign convention is defined for concave and convex mirrors.

v = distance of image formed.

u = distance of object.

In the NCERT books, the sign convention is like this: **Sign Convention for Reflection by Spherical Mirrors**

*New Cartesian Sign Convention* - In this convention, the pole § of the mirror is taken as the origin. The principal axis of the mirror is taken as the x-axis (X’X) of the coordinate system. The conventions are as follows –

- The object is always placed to the left of the mirror. This implies that the light from the object falls on the mirror from the left-hand side.
- All distances parallel to the principal axis are measured from the pole of the mirror.
- All the distances measured to the right of the origin (along + x-axis) are taken as positive while those measured to the left of the origin (along – x-axis) are taken as negative.
- Distances measured perpendicular to and above the principal axis (along + y-axis) are taken as positive.
- Distances measured perpendicular to and below the principal axis (along –y-axis) are taken as negative.

1/f = 1/v - 1/u

f= focal length. Know where to use focal length in positive and where in negative. The sign convention is defined for concave and convex mirrors.

v = distance of image formed.

u = distance of object.

**Sign Convention for Spherical Lenses**

For lenses, we follow sign conventions, similar to the one used for spherical mirrors. We apply the rules for signs of distances, except that all measurements are taken from the optical centre of the lens. According to the convention, the focal length of a convex lens is positive and that of a concave lens is negative. You must take care to apply appropriate signs for the values of u, v, f.

So you see, the positive and negative signs are all based on the side of the object / image.