Question

If the emissivity of an object varies with wavelength, it is called as __________

• A. \( {grey body} \)
• B. \( {black body} \)
• C. \( {selective radiant} \)
• D. \( {non-selective radiant} \)

Hint:

Selective radiators emit radiation differently at different wavelengths. This property is important in remote sensing, where sensors detect varying spectral signatures to differentiate materials.

Answer 1

The Correct Option is C

A selective radiant is an object whose emissivity varies with wavelength. In reality, most natural materials behave this way, emitting different amounts of radiation at different wavelengths depending on their physical and chemical properties.

In contrast:

A black body is an ideal emitter with emissivity equal to 1 at all wavelengths.
A grey body has constant emissivity less than 1 across all wavelengths.
A non-selective radiant is not a standard term in thermal physics.

Therefore, when the emissivity changes with wavelength, the correct term is selective radiant.