A double-convex thin lens such as that shown in Figure 3-21 can be used as a simple “magnifier.”
(a) What is its focal length in air?
(b) What is its focal length in water (n = 1.33)?
(c) Does it matter which lens face is turned toward the light?
(d) How far would you hold an index card from this lens to form a sharp image of the sun on the card?
(a) Use the lensmaker’s equation. With the sign convention given, we have ng = 1.52, n = 1.00, r1 = +20 cm, and r2 = − 15 cm. Then
So f = +16.5 cm (a converging lens, so the sign is positive, as it should be)
f = 60 cm (converging but less so than in air)
(c) No, the magnifying lens behaves the same, having the same focal length, no matter which surface faces the light. You can prove this by reversing the lens and repeating the calculation with Equation 3-11. Results are the same. But note carefully, reversing a thick lens changes its effect on the light passing through it. The two orientations are not equivalent.