79. We use the result of the problem 51 to solve for ψ. Note that φ = 60.0
◦
in our case. Thus, from
n =
sin
1
2
(ψ + φ)
sin
1
2
φ
,
we get
sin
1
2
(ψ + φ) = n sin
1
2
φ = (1.31)sin
60.0
◦
2
= 0.655 ,
which gives
1
2
(ψ + φ)= sin
−1
(0.655)= 40.9
◦
. Thus, ψ = 2(40.9
◦
)
− φ = 2(40.9
◦
)
− 60.0
◦
= 21.8
◦
.