1/1/92 - Why do things glow when they get hot?

## Why do things glow when they get hot?

(Lansing State Journal, Jan. 1, 1992)

Question submitted by J.P Joule of Holland.

All objects are made up of atoms, which vibrate when thermal energy is received from an outside source (for example, from the sun). The greater the thermal energy received, the more vigorously the atoms vibrate. One result of these vibrations is the emission of electromagnetic waves. The character of these waves is determined by the temperature of the object.

Electromagnetic waves are found everywhere. Ultraviolet (UV) and x-rays are examples of high frequency electromagnetic waves, while the waves which carry radio and TV signals have lower frequencies. Our eyes can see only a very narrow portion of all the electromagnetic waves; we recognize these waves as the colors from red to violet, with the color depending on the frequency. Violet, having the highest frequency of all visible light, is at one end of the color spectrum, while red is at the other end and has the lowest frequency. White is the sum of all colors. At slightly lower frequencies than red are infrared (IR) waves which are invisible to the human eye, but can be felt in the form of heat. The accompanying figure shows the electromagnetic spectrum and the common names used to identify different types of waves.

All objects emit electromagnetic waves. The temperature of the object determines which frequencies dominate, and thus which colors you see when you look at an object. Cooler objects emit primarily low frequency wavelengths, while warmer objects emit waves with primarily higher frequencies. As the temperature of an object increases, the increasingly energetic vibrations of the atoms result in the emission of correspondingly higher frequency waves. The human body emits waves which are of frequencies corresponding to the infrared range; we sense these rays as heat. Objects at higher temperatures emit waves of high enough frequency to be visible and we see these objects as being red. If the temperature of the object continues to rise, the emitted wavelengths become even higher in frequency, and the observed color becomes orange or yellow. At even higher temperatures, the amount of visible light emitted by the objects covers all of the visible spectrum about equally and our eyes see the objects as white, hence the term `white hot'.

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