Answer:

Step-by-step explanation:
The change in the wavelength observed due to the relativistic doppler effect when the source approaches is given by the following expression:

Here,
is the emmited wavelength (red),
is the observed wavelength (green) and v is the approaching speed. So,
and
. Solving for v:


Recall that
, replacing this and the wavelengths:

This is the incredible speed at which you would have to be moving to see the red light as green light.