The range of the wavelength for long wave infrared radiation is approximately

• A. 4000-8000 mu
• B. 4000-8000 A
• C. 800-1500 mu
• D. 1500-12,000 mu

Answer: (D)

Long wave infrared is the portion of the infrared spectrum where thermal radiation from everyday objects at typical temperatures is strongest. This region is characterized by longer wavelengths, roughly from about 1.5 micrometers up to around 12 micrometers, with many practical applications focusing on the 8–12 micrometer range where detectors and atmospheric transmission are favorable. Wien’s displacement law explains this nicely: a body at room temperature emits most strongly near about 9–10 micrometers, so long wave infrared detectors are designed to work in that neighborhood.

Among the options, the range that best matches this LWIR band is the one that spans from about 1.5 to 12 micrometers, since it covers the longer end of the infrared spectrum where thermal emission is prominent. The other ranges fall either into visible light, near-infrared, or far-infrared/microwave territory, which is why they aren’t the correct representation of long wave infrared.