Answer:
an object that is moving to the left or right (perpendicular from you line of sight) would not be moving closer or farther away from you and would therefore be neither red nor blue shifted.
Step-by-step explanation:
You can only use the Doppler effect to detect the radial velocity of an object because an object that is moving to the left or right (perpendicular from you line of sight) would not be moving closer or farther away from you and would therefore be neither red nor blue shifted.