Refractive index 
for radio waves
Refractive index is frequency dependent
Frequency dependant delay in arrival time
Dispersion Measure - DM
Scattering
Scintillation
Faraday Rotation
Figure 7: Dispersed pulses from the Vela pulsar. The bottom trace shown the
profile obtained by dedispersing and summing the other traces. The
asymmetric nature of the pulse profile is due to interstellar scattering.