Question

A particular 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?

Answer 1

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.