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