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2015-01-01T15:50:00+05:30

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To know when to put positive and negative sign, you need to learn the sign convention used for mirrors and lenses. You can refer to your book. It is clearly explained there. See the example to have a better understanding.
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2015-01-02T03:43:30+05:30

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I am writing here for lenses.   It will be similar for mirrors. 

The formula is always 
        1/v - 1/u = 1/f    and     magnification is  m = v/u = h'/h

Sign convention:

1.  Distances  u , v ,  f   measured against the direction of  rays from object are negative.  If object on the left, lens on the right side, then  u is negative.  Focal length for convex lens is +ve.  Focal length for  concave lens is negative.

2.  Distances or heights h or h' of objects and images which are erect (+Y direction) are positive. Inverted lengths h' or h  in -y direction are negative. 

Substitute +ve or negative sign along with numerical values.  

One good thing, you can do,  call  U, V, F, H, H' to be always positive and equal to |u|, | v | ,....

Then for a concave lens,  u = - U,  v = - V, f = -F.   h = H,  h' = H'
For a convex lens for real image,  h = H,  h' = -H'   f = F,  u = -U, v = -V.

Make a table of  all signs for cases of all virtual and real images for all lenses and mirrors.    Then only  you will understand it completely.


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