Question

A convex lens of a focal length f can be defined by the lens equation 1/f=1/p + 1/q if an object is a distance p from the lens, then the distance q from the lens to the images. For a particular lens, f=2cm and p is increasing find
1. a general formula for the rate of change of q w.r.t. p.
2. the rate of change of q w.r.t. p=22cm

Answer 1

a. 1/q=1/f-1/p
1/g= (p+f)/pf
q= pf/(p+f)
dq/dp= f/(p+f)-pf/(p+f)^2 (remember f is a constant, so df/dp=0)
check that.

Answer 2

is part 2 1/144