What wavelength of infrared (IR) light is completely invisible to the human eye?

The wavelength of infrared (IR) light that is completely invisible to the human eye is 940nm. This wavelength falls within the infrared spectrum, which ranges from approximately 700nm to 1mm. The human eye is typically sensitive to wavelengths between about 400nm and 700nm, meaning that any light outside of this range, including infrared light, will not be perceived by humans.

At 940nm, the IR light is near the upper limit of the infrared range and is widely used in applications such as night vision and remote sensing because it provides an effective balance of invisibility and detection capability. Many cameras and sensors are designed to utilize this specific wavelength since it minimizes the chance of detection by the human eye while still providing effective performance in various applications.

The other wavelengths listed, including 860nm, 980nm, and 850nm, fall within the near-infrared spectrum, but they may not be completely invisible to all human observers. For instance, 850nm may still be detected under certain conditions. Thus, 940nm is the most commonly accepted wavelength for IR that is effectively invisible to the human eye, making it the correct answer in this context.