Which option correctly lists the starting wavelength of the infrared range relevant to heat sensing?

• A. 400 nanometers
• B. 700 nanometers
• C. 1 micron
• D. 10 micrometers

Answer: (B)

Infrared begins where visible light ends, around the red end of the spectrum. Heat sensing relies on detecting infrared radiation emitted by warm objects, so the starting point is about 0.7 micrometers, which is 700 nanometers. The option at 400 nanometers is still visible light, not infrared. The 1 micrometer point is in the infrared, but it’s deeper into the near-infrared rather than the left edge. 10 micrometers is mid/far infrared, well past the starting boundary. So 700 nanometers is the correct starting wavelength for infrared relevant to heat sensing.