This week’s question was asked by a friend.
QUESTION: Why does iron turn a red color when you heat it?
It’s amazing, but true; everything glows. Objects are made of atoms and will glow at wavelengths (color) that depend on their temperature. Even objects at room temperature, say in our living room, glow. But that radiation is in the infrared region and invisible to the human eye.
At temperatures of several hundred degrees, objects begin to glow in the visible spectrum. The lowest energy part of the visible spectrum is red light, so that’s the first color we see. Light is made of waves, just like water waves, except we can’t see the individual waves as we can with water waves. Light consists of seven colors, easily remembered by using the memory aid ROYGBIV, which means red, orange, yellow, green, blue, indigo and violet.
Red is the lowest frequency (rate of vibration) and longest wave we can see, and violet is the highest frequency and shortest wave. Wavelength is, as the name implies, the length of each wave, say from crest to crest. Violet light has almost twice the frequency and half the wavelength of red light waves. When iron is white hot, it is radiating waves of all the colors put together, which, of course, is white light.
The temperature of any incandescent body, whether it’s a star, blast furnace or piece of iron, can be determined by measuring the frequency and wavelength of the light emitted.
Those old-time blacksmiths would heat up those horseshoes and other iron works. When the iron got hot enough, it would glow a dull red. As the iron got hotter, it would turn orange-yellow, then bluish and perhaps a bluish-white. The color or frequency is proportional to temperature. The higher the frequency, the higher the temperature.
Unseen during this heating process is that objects will also begin glowing in the ultraviolet region and at even higher temperatures will produce x-rays. The light from an arc welder is the glow from superheated material that the electric arc is passing through. That glow will include ultraviolet (UV) light, which can do damage to the eyes and skin. Hence, the need to wear protective eye wear when welding.
This whole business of metals giving off colors is called black-body radiation. When an object is heated up, the electrons and atoms get all excited, so the electrons jump up to a higher energy level. The electrons move to orbits further away from the nucleus. When the electrons go back to their ground or home state, the atom releases a bit of light, called a photon, which has a wavelength and energy that corresponds to how energetic the electron became.
Why don’t wood and paper glow? Answer; a chemical reaction gets in the way. A tungsten filament in those old-fashioned light bulbs gets so hot it glows fairly white. The coils in a stove unit get sufficiently hot to emit a reddish glow. The sun gets hot enough to glow yellow-white. Blue stars are hotter than red stars. The bright blue super giant star, Rigel, below the belt of Orion, the Hunter, has a surface temperature of 35,000 degrees Fahrenheit. The reddish star Betelgeuse, above the belt of Orion, has a surface temperature of only 5,800 degrees Fahrenheit.
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Larry Scheckel is a retired Tomah High School physics teacher.