Wavelength of Infrared radiation:

Question

Accepted Answer

The correct option is (3):
The generally accepted scientific range for Infrared (IR) radiation in the electromagnetic spectrum is **760 nm to 1 mm**. The Infrared spectrum is defined as the radiation that lies just beyond the visible red light. Since the visible spectrum ends around 700 to 780 nanometers (nm), 760 nm is a widely cited and officially recognized starting point for the infrared range, according to international standards such as those set by CIE and ISO. This range extends up to 1 millimeter (mm), which is where the microwave spectrum typically begins. This entire range is critical for physical education and therapy because IR radiation is experienced as heat and is the primary mechanism in thermotherapy treatments.

The incorrect option is (1):
The range of **1400 nm-1 mm** is too high as a starting point for the overall infrared spectrum. The total IR spectrum is commonly subdivided into near-infrared (IR-A), short-wavelength infrared (IR-B), and long-wavelength infrared (IR-C). The Near-Infrared region (IR-A) spans from the visible boundary (around 760 nm) up to 1400 nm. By starting at 1400 nm, this option incorrectly excludes the entire Near-Infrared band, which has important applications in fiber optics and night vision. Therefore, it does not represent the *total* wavelength range of infrared radiation.

The incorrect option is (2):
The range of **3000 nm-1 mm** is far too restrictive and inaccurate for the complete infrared spectrum. This starting point of 3000 nm (or 3 micrometers) excludes a very large and significant portion of the spectrum, specifically both the Near-Infrared (IR-A) and Short-Wavelength Infrared (IR-B) regions. In fact, 3000 nm is closer to the beginning of the Mid-Wavelength Infrared (MWIR) or the long-wavelength IR-C subdivision. By cutting out almost all the shorter-wavelength infrared bands, this option fails to define the full extent of the radiation.

The incorrect option is (4):
The range of **900 nm-1mm** is inaccurate because its lower limit is incomplete. While 900 nm is definitely within the infrared spectrum, the scientifically accepted starting point for the IR spectrum is closer to the boundary of visible red light, which is approximately 760 nm. The span between 760 nm and 900 nm, although small in absolute terms, is a vital part of the Near-Infrared (IR-A) region. Therefore, defining the range from 900 nm omits the first and shortest wavelengths of infrared light.