Question

A certain substance emits only the wavelengths ${\lambda }_1,{\lambda }_2,{\lambda }_3$ and ${\lambda }_4$ when it is at a high temperature. When this substance is at a lower temperature, it will absorb which of the following wavelengths?

• A. ${\lambda }_1$
• B. ${\lambda }_2$
• C. ${\lambda }_1$ and ${\lambda }_2$
• D. ${\lambda }_1,{\lambda }_2,{\lambda }_3$ and ${\lambda }_4$

Answer 1

The correct option is D ${\lambda }_1,{\lambda }_2,{\lambda }_3$ and ${\lambda }_4$

Let $e$ be the emissivity and $a$ be the absorptive power of the body.

From Kirchhoff's law,
$e=a$
It means, if a substance emits a certain wavelength at a high temperature, it absorbs the same wavelengths at considerably lower temperature.

From question, the substance emits only the wavelengths ${\lambda }_1,{\lambda }_2,{\lambda }_3$ and ${\lambda }_4$. This means that the substance will absorb the same wavelengths at lower temperature.

Thus, option (d) is the correct answer.