Answer & Solution
Answer: Option
B
Solution:
strong>Electromagnetic spectrum for thermal radiation extends over a part of the full EM spectrum where bodies at ordinary temperatures (like furnaces, heaters, process equipment) emit significant energy.
In heat transfer, thermal radiation is generally taken to be important over wavelengths from about 1 to 100 microns (μm). This range covers:
Shorter infrared waves near the visible region (around a few microns), where hot bodies at high temperature radiate strongly.
Longer infrared waves (tens of microns), important for bodies at moderate temperatures that are common in chemical and process industries.
Option A: 1 to 100 μm therefore represents the conventional engineering range used for radiation heat transfer calculations.
Why others are wrong:
Option B (0.5 to 50 μm) is too narrow; it misses part of the longer wavelength range where cooler surfaces still emit appreciable energy.
Option C (10 to 100 μm) ignores the shorter wavelengths (1–10 μm), where very hot surfaces have their peak emission.
Option D (100 to 1000 μm) lies mainly in the far infrared/microwave region and does not represent the main band of thermal radiation used in heat transfer.
So, the important radiation range in heat transfer is 1 to 100 microns, making Option A correct.
