When you leave warm shower, you’re glowing

When we humans look at most objects, the image we see is because of the available light (from the sun or artificial lights) reflecting off the object. If no light is available, we see nothing.

This wouldn’t be true if our eyes could see a wider range of wavelengths of light. Most objects give off light of their own, that is, they “glow.”

The wavelength of this light depends on the temperature of the object. For objects at room temperature, most of the glow is at wavelengths that are longer than our eyes can see, in the range from roughly 5 to 35 micrometers.

Our eyes respond to wavelengths in the range from about 0.45 to 0.65 micrometers. This is the range of wavelengths for the sunlight that reaches the earth; so our eyes are tailored to see reflected sunlight, rather than the glow from things that are near room temperature.

We do see the glow from objects that are much hotter than room temperature. The most common example is the filament in light bulbs. They operate at temperatures of several thousands of degrees precisely so that we can see them.

We’ve designed them to mimic the sun, which has an apparent temperature on Earth of around 5,500 degrees centigrade (9,000 degrees F.).

If we could “re-tune” our eyes to see longer wavelengths, we would see a strange world at night where warm things (people, animals, car exhausts, furnace vents, etc.) would glow brightly.

Things would appear much as they do through night vision optics, such as the military uses, and can be purchased (mostly by hunters).

The peak wavelength of the glow depends on the temperature of the object, as previously mentioned. It is inversely proportional to it, that is, if the absolute temperature is doubled, the wavelength is halved.

For temperatures in degrees Fahrenheit, we must add about 460 to that temperature to get absolute temperature, which would be in degrees Rankine. You might note that if the temperature was reduced to zero, the wavelength would become infinite. Scientists have been trying to reach absolute zero for many years; they can come very close, but have yet to attain it. As is predicted, the wavelengths become very large.

The total optical power in the glow varies as the fourth power of absolute temperature; that is, if the temperature is doubled, the power increases 16 times.

This is called the “Stefan-Boltzmann” law. This power also depends on how reflective (that is, how “shiny”) the surface is. Surfaces that look black emit the most light and mirror-like ones the least. When we consider that the glow represents power (and therefore energy) leaving an object, we are tempted to ask ourselves, why aren’t all these things cooling off if they are losing energy?

The answer is that all objects also absorb optical energy. If we walk out in the sun, we absorb more energy than we give off, and we feel warmer. The warmer we get, the more energy we give off until a balance is reached where the absorbed energy and the energy in the glow (emitted energy) are equal. Our temperature then is stable.

So why do we care about this?

Not only does it allow us to do night vision, but it makes possible optical thermometers so that we can rapidly spot temperature differences in many objects, such as hot spots in engines and electronics or hot regions of the earth’s surface (such as budding volcanoes) using satellites. It allows us to see immense distances in space (such as with the Hubble telescope). We even use it to rapidly screen animals and humans for sickness by looking for the hot individuals in herds, flocks and crowds.

So when you come out of a warm shower in the morning and feel all aglow, you are.

Phillip Jessen holds a degree in electronic engineering. He has designed a wide range of systems, including radars, nuclear instrumentation and space electro-optics.


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