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

More In: Science | Sci-Tech