Physics 102 glass converging

Question

A glass converging lens (n=1.50) is designed to look like the lens.

The radius of the first surface is 15 cm and the radius of the second surface is 10 cm. a)find the focal lenght of the lens.

Determine the position of the image for object distances of b)infinity

c)3f d)f and e)f/2

Answer 1

The lensmaker's equation (ref.) says:

1/f = (N-1)*(1/R1 - 1/R2), where R1 is positive and R2 is negative for a double-convex lens.

This is a thin-lens approximation of the full formula, which accounts for the distance between the two sides. Since this distance isn't known, we use the approximation.

Solving, f = 0.12 m

B. di = 1/(1/f-1/inf) = f

C. di = 1/(1/f-1/(3f)) = 1.5f

D. di = 1/(1/f-1/f) = undefined

E. di = 1/(1/f-1/(f/2)) = 1/(1/f-2/f) = -f. Since di is negative the image is virtual, on the same side of the lens as the object.