JD wrote:
". . .
f) The oscillating electrons radiate. If you work out all the
phases, including the equation of motion for the electron, and
the radiation from a moving electron, you find that the new
contribution to the field is nearly 90° out of phase with the
incident light, and smaller in magnitude. If you add this to
the original incident light, you get a resultant that slightly
lags the incident wave. If you don't believe me, simulate
it using a spreadsheet. Add a unit-sized sine plus about
20% of a cosine and see what you get.
Hint: https://www.av8n.com/physics/img48/sine-shift-sine.png . . ."
The Sine wave, the Cosine wave and the resultant wave in this picture all
travel with the same velocity. They are only out of phase with each other.
A Sin(kx-wt) +B Cos(kx-wt) = RSin (kx-wt+Theta)