Your Turn on page 134 claimed that, in one-dimensional diffusion, an ob- server sitting at a fixed point sees a transient pulse of concentration pass by. Make this statement more useful, as follows: Write the exact solution of the diffusion equation for release of a million particles from a point source in three dimensions. Then show that the concentration measured by an observer at fixed distance r from the initial release point peaks at a certain time. a. Find that time, in terms of r and D. b. Show that the value of the concentration at that time is a constant times and evaluate the constant numerically. -3