Question

why do the room heater have a polished metal surface behind the heating element

Answer 1

it is because metal is a good conductor of heat. thus, the heator would provide heat only when it comes contact with electricity (current. )

Answer 2

Electric room heaters are provided with a curved,polished surface behind the rod like heating element.this polished surface reflects back all the heat radiations falling on it ,concentrating it on forward direction.so that we feel even the heat even if stand a few feet away from the heater.