Write an expresssion for doppler shift.

Explanation

When the speed of source is small compared to that of light, the expression for doppler shift is giben by
`v_("radical")/(c)=+-(Deltav)/(v)=+-(Deltalamda)/(lamda)`
where `(Deltav)/(v)` is the fractional change in frequency and `v_("radical")` is the source velocity.
When source and observer approach each other, then `Deltav` is positive and `Deltalamda` is negative i.e. apparent frequency increases or apparent wavelength decreases. This is called blue shift. The reverse is called red shift.