Trucks often have signs on their backs that say, "If you can't see my mirrors, I can't see you." Explain the physics here.

Question

Trucks often have signs on their backs that say, "If you can't see my mirrors, I can't see you."  Explain the physics here.

drwls · Accepted Answer

The only way a truck driver can see a car behind is with the side view mirrors. If you cannot see either side view mirror of the truck, then there is no way for the driver to see you, because a ray of light must be able to pass from your car to a side view mirror.