Question

If a black wire of platinum is heated, then it's colour first appears to be red, then yellow and finally white. This observation can be understood on the basis of

• A. Wien's Displacement law
• B. Newton's law of cooling
• C. Expansion on heating
• D. None of these

Answer 1

The correct option is A Wien's Displacement law

From Wien's Displacement law,
${\lambda }_{m}T=b;$ where ${\lambda }_m$ is the
wavelength of maximum intensity (peak wavelength) for radiation at temperature $T$ and $b$ is a constant.

$\Rightarrow {\lambda }_m\propto \frac{1}{T}$

This law tells us that as the temperature increases, the
peak wavelength decreases.

On heating the black platinum wire, temperature increases and thus the peak wavelength decreases because of which the colour of the wire changes from red $(\lambda =700\text{nm})$ to yellow $(\lambda =600\text{nm})$ to white.