# Problem P. 4142. (February 2009)

**P. 4142.** An object is at a distance of *d* from a screen. The convex lens, placed between them, forms the sharp image of the object at the screen at two different positions, the distance between these positions is *s*.

*a*) Find the focal length, *f*, of the lens.

*b*) Suppose that *d* is known very precisely. When will the uncertainty in the measurement of *s* cause a greater error in the calculation of *f* if the value of *s* is greater or if it is smaller?

(4 pont)

**Deadline expired on March 11, 2009.**

### Statistics:

91 students sent a solution. 4 points: 62 students. 3 points: 7 students. 2 points: 13 students. 1 point: 4 students. 0 point: 3 students. Unfair, not evaluated: 2 solutions.

Problems in Physics of KöMaL, February 2009