70. We apply Eq. 34-42 (twice) to obtain
I = I
0
cos
2
θ
1
where θ
= 20
◦
and θ
= (20
+ θ). Since I/I
= 0.200,we find cos θ
=
√
0.2265 which leads to θ
= 62
and consequently to θ = 42
.