Why do all metals turn red when heated?

M. HARSHAN, Chennai

Heated bulk metallic objects turning red should be differentiated from some metals showing different colours when burnt either as vapours in a flame or as fine powder particles, wafers, shavings ex: magnesium fine powder/tape used in sparklers generating lot of brilliant yellow flashes. The former is described by the process of incandescence while the latter can be described using several intra-atomic electronic transitions.

Basically incandescence is a process of generating glow-light associated with heating. The popular examples being burning of wood and a glow from a conventional tungsten lamp. In the lamp, electric current passing through the metallic tungsten filament encounters resistance for the flow of electrons which causes lot of Joule heating appearing as glow-light of the filament.

A bulk metallic object when heated reaches high temperature and starts glowing. For any hot body, the colour of the glow can be used as a measure of its body temperature, known by the term correlated colour temperature (T) given in relation to black body radiation with the latter being the perfect emitter.

It should be borne in mind that the colour temperature is more a measure of glow/emission colour which can be related to that of the black body when its temperature is raised to that indicated temperature. The correlated colour temperature (T) of a hot body and the colour of the glow — emission wavelength λ (lambda) bear inverse relation through a law known as Wien’s displacement law. That is warm colours (yellow/red) having longer wavelengths have lower colour temperature T, while cool colours ( towards blue) show higher T

As an example, a yellow glow from a tungsten lamp or burning of candle show colour temperatures T in the range of 2,500-3,500 K, degree Kelvin while a blue light or /sky has a colour temperature of 10,000 K.

Obviously, even a red colour emission from a heated metallic object calls for a very high temperature ( approximately 2,000 deg C) which is closer to /surpassing melting points of most of the common metals available. Turning to explain the absence of cool colour glow from heated metals (that is shifting towards blue emission) calls for still higher colour temperatures which are many times higher than the melting points of metals. But further raising the temperature would readily render the most of metals vaporized /unstable.

R. JAGANNATHAN, Luminescence Group, CECRI Karaikudi, Tamil Nadu

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