In first case refraction of the rays, takes place from a plane surface, so we can use dapp =
dactual
µ
3=
4
µ
⇒µ=
4
3
where, dapp and dactual = apparent and real depth, respectively. Now, in second case, as from Fig. (b), refraction takes place from a spherical surface, so
µ2
v
−
µ1
u
=
µ2−µ1
R
or
1
(−25∕8)
−
4∕3
(−4)
=
1−4∕3
−R
or
1
3R
=
1
3
−
8
25
=
1
75
R=25cm Now, using Lens’ Makers formula, to calculate focal length