To complete the specification of the solution at any point 
,
we need not only the amplitude, but also the phase. For waves
traveling out in the radial direction away from the point source, we
expect a form just like the string, 
,
where 
is some phase. Given what we know of the amplitude
from Eq. 3, the solution at any point at
distance 
from the slit is then
Note that, we here have written the answer also in complex form,
which
describes simple harmonic oscillations of complex amplitude
This will be useful in future sections.
