1. The problem statement, all variables and given/known data a) In the autocollimation experiment of Sec. 3.5, a thin convex lens is placed 25 cm from a pinhole that acts as a point source of light. A plane mirror placed 20 cm behind the lens reflects the light back through the lens, and the reflected light forms a sharp spot beside the pinhole. What is the focal length of the lens (in cm)? b) Consider a symmetrical biconvex lens with r1 = r2 = r How would you alter the lens if you wanted to make the focal length longer? Make it flatter (more like a flat slide) or make it rounder (more like a sphere)? A. 1/f = 0 B. 1/f = 2(n - 1)/r C. 1/f = 2(1 - n)/r D. 1/f = (n - 1)/r 2. Relevant equations 1/o+1/i=1/f (n-1)(2/r)=1/f 3. The attempt at a solution a)I used the lensmaker's equation but the answer was wrong. Shouldn't this formula apply? b) Increasing the focal length would occur if the radius was increased. Since it is a biconvex lens, they will increase if the lens is made to go outward more, such as if it was a sphere instead of flat.