Question

in mirror formula which sign remain always positive​

Answer 1

In the case of spherical mirror all ... Sign is taken as – (negative) from pole of a spherical mirror towards object along the principal axis.

Answer 2

Mirror formula --

1/f=1/v+1/u....

Object distance(u) always remains '-'(negative)

focal length(f) of concave mirror is negative and that of cinvex mirror is positive..

image distance(v) in concave mirror is negative if image is real and invertes and positive if image is virtual and erect..

In convex mirror image distance(v) is positive....

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