Question

how can we know the which sign is to be there in concave mirror that whether u is negative or positive always and also similarly convex mirrors and a also lens

Answer 1

★ LENSES AND MIRRORS ★

According to classical generality in optical physics , we withstand the portion available for practical aspects is termed negative [ on the left side ] , and behind them is stated as positive [ on the right side ]

Aslike -

Direction in front of mirror from which incidence ray falls on the surface is termed as negative [ on the left side ] , height above the principal axis is termed as positive on the same side keeping in mind during mirrors [ concave and convex ]
and spherical ones ,

The same is applicable in lenses ,
Sign convention in lenses goes like -

Right side of the lens is positive
left side is negative
above the principal axis is positive , irrespective of side ,
below is stated as negative

Dealing individually each case is not possible , so , putting up above the General format of sign conventions in both the lenses and mirrors

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Answer 2

                         Note :

• f = focal length

• v = distance of image .

• u = distance of object .

Lens formula :

1 / f = 1 / v - 1 / u

Concave lens :

f = negative

u = negative

v =  negative

Convex lens

f = positive

u = negative

v = positive for real image ,

v = negative for virtual image .

The optical centre is chosen as the origin of the coordinate system.

With respect to optical centre :

The part which are above are positive, below negative.

The part which are right are positive while left is negative.

This is just like the normal x-axis and y-axis in the co-ordinate system .