When do u take image distance positive or negative?

When do u take object distance negative or positive?

Plzz explain it in detail

The following formula apply to both mirrors and lenses:

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

The greatest difficulty is in remembering the signs of the variables. I think the easiest way to do it is the following:

1. u is always positive
2. v is positive for a real image, which is of course in front of a mirror, or behind a lens.
3. f is positive for a converging lens or mirror.

All of the possibilities are listed in the following table. I think this is an unnecessarily complicated way of summarizing the different cases; but some students prefer to have it stated this way:

	focal length	object distance	image distance
concave mirror	positive	positive	positive, if on the same side as object (real) negative, if on the opposite side as object (virtual)
convex mirror	negative	positive	negative,as all images are virtual and on the opposite side as object (virtual)
converging lens	positive	positive	positive, if on the opposite side as object (real) negative, if on the same side as object (virtual)
diverging lens	negative	positive	negative,as all images are virtual and on the same side as object (virtual)

Truths for both lenses and mirrors

1. Image distances are always negative for virtual images and positive for real images.

2. Object distances are always positive.

3. Real images are always inverted and virtual images are upright.