How Radiant Heat Works

Buckley Rumford Fireplaces
How Infrared Works

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Transparency (from Univ of Georgia Physics Dept.
You can see for many miles through clear air and a clear piece of glass obviously is transparent to the wavelengths of visible light. The air is fortunately not transparent to the ultraviolet rays from the sun, though increasing transparency from ozone depletion is a concern. The clear piece of glass is transparent to visible light because the available electrons in the material which could absorb the visible photons have no available energy levels above them in the range of the quantum energies of visible photons. The glass atoms do have vibrational energy modes which can absorb infrared photons, so the glass is not transparent in the infrared. This leads to the greenhouse effect. The quantum energies of the incident photons must match available energy level gaps to be absorbed.

Greenhouse Effect
Planck's Law of Black Body Radiation describes radiant energy as a function of wave length and, according to Wien's Law, wave length is inversely proportional to temperature. Finally, from Kirchhoff's Law and our discussion about transparency, we understand that different materials have different properties of absorption, emissivity and transparency based on their molecular structure. Glass, for example, is transparent in the visible wave length range of about half a micron but is opaque in the infrared wave length range of from one to two microns.
Now we can see how a greenhouse works. Nearly all the energy from the sun where the temperature is about 6,000 K, passes through the glass at a wave length of about one half micron (yellow line) but the heat re-radiated inside the green house at room temperature of about 80 deg. F (we call 300 K radiation) emit radiation which peaks at about 9.6 microns, which glass does not transmit. Thus the emitted energy from the inside objects in a greenhouse does not get out.
So do you imagine that glass doors would improve the efficiency of a fireplace? See The Skinny on Glass Doors for the answer.