Rules for using one mirror or lens.
A concave mirror is a converging mirror, and a convex lens is a converging lens.
However, a convex mirror is a diverging mirror, and a concave lens is a diverging lens.
Note that concave and convex change with respect to convergence/divergence when
changing from mirrors to lenses.
A converging mirror/lens has a positive focal length f; a diverging mirror/lens has a
negative focal length f. Also, for a mirror, the focal length f is one-half of the radius
of curvature R of the mirror.
In general, we have the formulas
1 1 1
 
s s f
and
m
 s  h

s
h
We will always have a positive object distance s, but s’ can be positive or negative.
If s’ is positive, then we have a real image, whereas if s’ is negative we have a virtual
image.
This means that, given our definition of the magnification m, that a real image always
has a negative magnification, meaning it is inverted, whereas a virtual image always has
a positive magnification, meaning it is upright.
So, for our simple systems, an image is either real and inverted, or virtual and upright.
We can see both real images and virtual images, but a real image can be focused onto a
screen or a piece of photographic film, but a virtual image cannot.
A plane mirror has R = ∞, so f = ∞, s’ = –s, and m = +1. So the image from a plane
mirror is always virtual, upright, and the same size as the object.
For one spherical mirror or lens: (These rules don’t work for more than one mirror or
lens, but we won’t worry about those cases.)
If the object is more than the focal length away from a converging mirror or lens,
you get a real image.
If the object is less than one focal length away from a converging mirror or lens,
you get a virtual image that is bigger than the original object. (Magnifier!)
If the object is in front of a diverging mirror or lens, you will get a virtual image
that is smaller than the original object.