For object and image at infinity the separation = sum of focal lengths

In air separation is 50 + 15 = 65 cm.

Objective lens:

1/f = 1/50 (n - 1)(1/R1 - 1/R2) = (1.6 - 1)(2/R) since |R1|, = |R2| = R

1/50 = (0.6 X 2)/R

R = 60 cm

One surface of the objective in water:

P = P1 + P2 = (n2 - n1)/R + (n3 - n2)/R = (1.6 - 1)/60 + (1.33 - 1.6)/(-60)

= 0.01 + 0.0045 = 0.0145

f = 1/P = 1/0.0145 = 69 cm

Separation for object and image at infinity = sum of focal lengths = 69 + 15 = 84 cm

Return