Physic Lab question

Question

1. Using the experimental value of f=15cm for the biconvex converging lens and n = 1.5, compute the radius of curvature of the len's surfaces using the lensmaker's equation.

 

2. A student incorrectly assumes that f = R/2 for the lens and computes f using the value of R found in part (a). Compare this computed value of f with the experimental value.

f = 15cm

R = 30cm.

Answer 1

Given a biconvex converging lens, we have that R = R1 = R2, so the lensmaker's equation simplifies to

1/f = (n-1)(2/R)

1/f = (1/2)(2/R)

1/f=1/R

f=R

So we have R is 15 cm for the experimental value.

Now given the erroneous assumption that f_computed=R/2, we would have f_computed= 15cm/2 = 7.5 cm.

This gives f_experimental / f_computed = 0.5, or exactly half the experimental value.