Summary of mirrors and lenses


The mirror/lens equation:



The magnification equation(s):






For a mirror or lens, the light rays come into the optical device from one side; let’s call this the “IN” side.  The light rays leave the mirror or lens on the “OUT” side.  For a mirror, the “OUT” side is the same side as the “IN” side; for a lens, it’s the opposite.


Object on the “IN” side

dobj > 0

          (not on the “IN” side)

dobj < 0



Image on the “OUT” side

dimg > 0

          (not on the “OUT” side)

dimg < 0



Focus on the “OUT” side

f > 0     (concave mirrors, converging lens)

          (not on the “OUT” side)

f < 0     (convex mirrors, diverging lens)





dobj, dimg > 0:   REAL object, image

dobj, dimg < 0:   VIRTUAL object, image


M > 0:            UPRIGHT

M < 0:            INVERTED


| M | < 1:          SMALLER

| M | = 1:          SAME SIZE

| M | > 1:          LARGER


You only get “virtual objects” if the object for one lens (or mirror) is the image from another lens (or mirror), which is arranged so that dobj < 0 for the 2nd imaging device.  See section 26.9, which will be discussed Monday March 26.