Physlet Illustration: Focal Length of a Lens

 

 
In this simulation, a source of parallel rays is placed in front of a lens. The lens is located at x = 3.0 cm. What is the focal length of the lens?

Hints:

  1. Do the parallel rays converge at a point after they emerge behind the lens?
  2. If so, how far is this "focal point" from the lens?
  3. Do the rays, on the other hand, diverge after they pass through the lens?
  4. If so, do the emerging rays "appear" to emanate from a point in front of the lens? How far is this "focal point" from the lens?
  5. What, then, is the lens's focal length? (Be careful to get the sign correct.)

Reference

See Walker, Section 26-6

Illustration written by Steve Mellema