Aparticular spectral line from an observed object is measured to be at a wavelength of 400 nm. that same spectral line is found to have a wavelength of 800 nm when measured at rest in the lab. what is the radial velocity of the observed object?

To find the radial velocity of the object we are going to apply the regular Doppler formula:
 [measured wavelenght - rest wavelenght/ rest wavelenght] = V/C, where V is the radial velocity and C is the speed of light which is equal to 300,000km/sec.
[400 - 800/800] = V/300,000 = - 150,000.
This means that the object is moving at a velocity of 150,000km/sec toward the observer.
The minus sign infront of the answer indicates that the object is moving towards the observer while a positive value will indicate that the object is moving away from the observer.