Infrared radiation is often confused with warmth but although there is a strong relation
between them, they concern two different concepts. Warmth is a physical
phenomenon that deals with the temperature of a subject or of a medium that
surrounds it, for instance water or air. By convection this warmth can be transferred
to the subject, whereby the air or water acts as the transportation medium. In vacuum
this way of transportation is not possible. This is why in space the shadow side of a
large and (relative) cold subject remains cold despite the sun is shining unhindered.
An object that is heated up for some reason or another, starts emitting infrared
radiation and finally, when the temperature reaches values between 500 to 1000
degrees Celsius, also light. This infrared radiation behaves like light. It does not
require any medium to spread and it propagates rectilinear. An infrared radiator like
the sun is therefore capable of warming a subject in vacuum but only when this
subject is exposed to its radiation directly and not when it remains in the shadow. On
earth an infrared source will, through radiation, warm an exposed subject directly.
Since there is usually is a heat-conducting medium like air at present, the infrared
source, as well as the subject heated by it, will, through convention, also transfer a
part of their energy to the surrounding medium. This causes a rise of temperature of
the medium and that is where the relation between environmental warmth and infrared
radiation emerges.