Question

The thin lens formula in physic is given by 1
Do
+
1
Di
=
1
f
where Do distance of an object from the lens, Di distance of the
image from the lens, f is focal length of the lens
Suppose the object is moving away from the lens at rate of 3cm/s.
How fast and in which direction will the image be moving?

Answer 1

Do + 1/Di = 1/f ?
no way
1/Do + 1/Di = 1/f
if Do gets bigger then 1/Do gets smaller
Di has to get smaller so 1/Di gets bigger