98 ´ 10–19 J
(b) If N is the number of photons emitted by the source per second,
the power P transmitted in the beam equals N times the energy
per photon E, so that P = N E Then
N = 
3
19
2 0 10
W
3 98 10
J
P
E
−
−
×
=
×
         = 5