Do > 0
if diverging rays are incident on the lens
Do < 0
if converging rays are incident on the lens.
(This occurs only in multi-lens systems.)

 IMAGE DISTANCE

Di> 0 - image is real and found on right side of lens.
Di<0 --- image is virtual and found on left side of lens

 RADIUS of CURVATURE

R1 = radius of first surface encountered by light
R2 = radius of second surface encountered by light.
Convex surfaces are positive.
Concave surfaces are negative

 IMAGE FORMATION

REAL IMAGE
- formed only with a converging lens
and only if Do > f

VIRTUAL IMAGE
- formed by a converging lens if Do < f
-formed by any diverging lens