Why do all metals turn red when heated?
M. HARSHAN, Chennai
Materially, many of the hard metals are considered as ‘blackbodies'. All those materials that are perfect absorbers of all wavelengths of light when they are cool and emitters of all wavelengths of light when they are heated, are known as blackbodies. However, the wavelength (or colour) of the light they emit with greatest efficiency (maximum light throughput) varies with the temperature that they are heated to or maintained at.
Let us know this wavelength as ‘lambda max'. This means that a blackbody body heated to a particular temperature would appear in the colour of the lambda max wavelength because it is at that wavelength that the optical energy is emitted from the body with maximum throughput.
The ‘lambda max' and the temperature at which the blackbody is heated to are inversely related; the hotter the black body the lower is the lambda max. This is a universal law, known as ‘Wien's Displacement Law' and is independent of the chemical composition and physical fabric of the solid body as long as the body behaves as a blackbody when heated.
In other words, the mathematical product of the lambda max and the absolute temperature of the body is a universal constant, known as Wien's Constant with a value of about 2.9x10 mK when wavelength is taken in metres (m) and the temperature in Kelvin scale (K).
Accordingly, metals, behaving like blackbodies, would appear in red colour (wavelength of about 700 nanometres) when heated to about 3,800 K (or about 3,500 C).
It is on this basis (Wien's Displacement Law) that metals turn red when heated (to about 3,500 C). It is also the reason why metal objects exposed to blacksmith's kiln change their appearance from red to yellow via orange because of gradually raising temperature of the kiln.
Finally, it is worthwhile to know that the surface temperatures of the distant stars and heavenly bodies are estimated on the basis of the spectrometric measurement of the lambda max they emit (after accounting for the Doppler Shift). Since Sun appears yellow, its surface temperature is estimated to be about 5,500 C.
PROF. A. RAMACHANDRAIAH
Editor, Vidyarthi Chekumuki
Jana Vignana Vedika, Andhra Pardesh
Keywords: heated metal, red hot metal
It is refreshing to find a scientifically accurate answer in this S&T column.
In connection with your paragraph on "surface temperatures of the distant stars and heavenly bodies", may I add for information of readers that the same concept is applied in reverse for describing "colour temperature" of the display in computer monitors.
This article is worse than atrocious, specially keeping student readers in mind, because they are misled and mistaught by this article. I mean no offense to Prof. Ramachandraia whom I normally hold at respect for his knowledge. But this article is written with out much thought. There are lot of mistakes, one pointed out would prove it well. Iron has a melting point of 1535 degree Celsius which is 1808 Kelvin. The blacksmith works on iron at temperature far below this value and the iron appears red. So, how can we accept that the temperature corresponding to red colour is 3500 degree Celsius? I accept that the value of Wien's constant as given 2.9x10mK in stead of 2.9X10^-3mK could be a slip, but the students are at the loss.Sun appearing yellow (!) is equally a dangerous blunder, especially attributing a temperature on the basis of this colour. Sun light is taken by all standards as white light. My suggestion : The Hindu can be more careful in publishing such articles.
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