If ‘u’ is the object-distance, ‘v’ is the image-distance and ‘f’ is the focal length of a spherical mirror then which of the following is a correct expression for the mirror formula?
1
1/v + 1/u = –1/f
2
1/v – 1/u = 1/f
3
1/v + 1/u = 1/f
4
1/v – 1/u = –1/f