68.
(a) The radius of the central disk is
R =
1.22f λ
d
=
(1.22)(3.50 cm)(515 nm)
3.00 mm
= 7.33 µm .
(b) The average power flux density in the incident beam is
P
πd
2
/4
=
4(5.00 W)
π(3.00 mm)
2
= 707 kW/m
2
.
(c) The average power flux density in the central disk is
(0.84)P
πR
2
=
(0.84)(5.00 W)
π(7.33 µm)
2
= 24.9 GW/m
2
.