When an iron bar is heated to a very high temperature, it initially glows red, and then as its temperature rises, it glows white. This process is called incandescence. Incandescence is heat made visible when heat energy turns into light energy. Why does this happen?
Everything around us gives off both heat and waves of light called infrared light, which we cannot see. Even things that we think of as being very cold, like an ice cube, gives out some heat. As an object becomes warmer, its atoms emit much more wave energy, and the waves it produces become shorter and shorter. if it is warmed enough, the shorter waves will give off visible light as the object starts to glow.
When an object heats up, first it glows red, and then white when it becomes really, really hot. to put it in a nutshell, everything glows, not just hot objects.
Most things glow in the infrared frequencies, which human eyes can't see. When an object gets hot enough, it glows in the visible light frequencies, which we can see, and then we say that it is 'red-hot' or 'white - hot'!