If black wire of platinum is heated, then its colour first appear red, then yellow and finally white. It can be understood on the basis of
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(a) According to Wein’s law ${\lambda _m}T$= constant, on heating up to ordinary temperatures, only long wavelength (red) radiation is emitted.
As the temperature rises, shorter wavelengths are also emitted in more and more quantity.
Hence the colour of radiation emitted by the hot wire shifts from red to yellow, then to blue and finally to white.
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