ASK YOUR SCIENCE TEACHER | News
QUESTION: We saw a farrier heat up horseshoes at Old World Wisconsin, and he turned red. Why is that?
ANSWER: It was one of the great mysteries of science. Indeed, why should iron shine red or any other color. The answer to this question led to the birth of quantum mechanics or quantum physics.
When we heat a piece of metal, we add heat energy to the atoms inside the metal. This causes them to vibrate and they begin to emit electromagnetic radiation.
The blacksmiths of old heated these horseshoes and other iron works. When the iron was hot enough, around 1,470 degrees Fahrenheit, it glowed a dull red. As the iron got hotter, it turned orange-yellow, then bluish and possibly a bluish-white. Keep adding heat and the iron melts. Color or frequency is proportional to temperature. The higher the temperature, the higher the frequency.
Light is made of waves, just like water waves, except we cannot see individual waves like we can with water waves. Visible light is made up of seven colours, easily remembered using the ROYGBIV cheat sheet, which are red, orange, yellow, green, blue, indigo and violet.
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Red is the lowest frequency wave (rate of vibration) we can see, and blue, indigo and violet are the highest. Violet light waves have almost twice the frequency of red light waves. When the iron is white-hot, it emits waves of all the colors together, that is to say white light. The temperature of any glowing body, whether stars or blast furnaces, can be determined by measuring the frequency (color) of the light they emit.
It is interesting to think of objects, like iron, below the temperature at which they emit visible light. They emit invisible rays of infrared radiation, which are similar in nature to light. But they don’t contain enough energy to stimulate the optic nerve to be seen by the human eye. There are cameras that can “see” infrared, many are used as surveillance cameras, home security systems and by the military.
In now-old-fashioned incandescent bulbs, a filament is heated to a temperature at which a fraction of the radiation falls into the visible spectrum. The majority of radiation is emitted in the infrared part of the spectrum, which makes the incandescent bulb relatively inefficient as a light source. More efficient light sources, such as fluorescent lamps and LEDs, do not run on incandescence.
We see this relationship between color and temperature by looking up at the sky.
Observe the constellation of Orion, the Hunter. The hot young supergiant below Orion’s belt is Rigel, with a surface temperature of 19,000 degrees Fahrenheit. It is a beautiful bluish color. The old red supergiant star above the belt is Betelgeuse, with a surface temperature of 5,000 degrees Fahrenheit.
Classical physics predicted that hot objects would instantly radiate all their heat in electromagnetic waves. The “ultraviolet catastrophe” was the name given to this conflict between theory and observation. This contradicted the principle of conservation of energy and showed that a new model was needed to explain the behavior. It happened in the 1890s. It was one of the things that led to the invention of quantum mechanics. The German physicist Max Planck (1858-1947) solved the problem by proposing that electromagnetic energy is emitted in quanta with the equation E=hf. Quantum theory accurately predicted the energy of the observed radiation.
Max Planck led a long and useful life, filled with scientific triumphs and personal tragedies. His first wife died young. He was a Lutheran churchwarden from 1920 until his death and believed in an almighty, omniscient and beneficent God. Max Planck expressed the opinion that God is present everywhere.
During World War II, the Allied bombardment of Berlin forced Planck and his wife to leave the city and live in the countryside. In February 1944, their house in Berlin was completely destroyed. In late 1944, Planck’s son Erwin was arrested by the Gestapo following the attempted assassination of Hitler in the July 20, 1944 plot. He was tried and sentenced to death in October 1944 and hanged in January 1945. The death of his son destroyed Planck’s will to live. He died on October 4, 1947 and his grave is in the cemetery of the town of Goottingen. Max Planck’s name is on everyone’s list as one of the greatest scientists in history.
Sources: nasa.gov, scienceexchange.caltech, Famousscientists.org/list.
Larry Scheckel is a retired physics teacher from Tomah High School.