Iron goes red when heated because its atoms radiate vibratory waves of an electrodynamics nature which are visible as light at a sufficiently high temperature. At 800 Centigrade the iron is at low-red heat. But as the heat increases the iron will turn bright red, and finally white hot and molten.
Heat is passed through the iron by conduction the contact of one iron particle with another with no visible movement of the particles. The heat which is given off as light when iron glows red hot can be reconverted into heat by the substance on to which it falls. When iron is heated to a temperature below 300 Centigrade it gives off invisible rays of infra red radiation which are similar in nature to light. But they do not contain quite enough energy per unit to stimulate the optic nerve and so be seen by the human eye.