why do qbacks have a dot on back of helmet

They have a radio transmitter in their ear.

its a green dot on the back of the helmet, which recieves a radio signal for side line communications used for the earpeice inside the QB is helmet, the couch can only relay a play but the QB cant reply back

to easily identify them,,,

its to let the refs know who has the electronic communication with the coaches on the team

Mostly, for obvious reasons, QBs will have a green dot on their helmet. It signifies that the player has a radio helmet & the NFL will put the stickers on the helmets themselves each week.

Only 1 green dot can be on the field at a time. So, if a 2nd string QB comes in for a trick play, he has to have a different helmet to wear.

There can be many green dots, but only one can be on the field at once

Isnt tis thw wrong section? And the answer is it is there so the zebras know who has the radio in the helmet