Formula review
The following two formulas apply to both mirrors and lenses:
The greatest difficulty is in remembering the signs of the variables.
focal length |
object distance |
image distance |
|
concave mirror |
positve |
always positive |
positive, if on
the same side as object (real) |
convex mirror |
negative |
always positive |
negative,as all
images are virtual |
convex lens (converging) |
positive |
always positive |
positive, if on
the opposite side as object (real) |
concave lens (diverging) |
negative |
always positive |
negative,as all
images are virtual |
Truth's for both lenses and mirrors
Image distances are always negative for virtual images and positive for real images.
Object distances are always positive.
Real images are always inverted and virtual images are upright.