Thermal scopes are optical devices that measure heat radiation, which essentially causes the object to glow under infrared or thermal imaging. The unaided human eye cannot see on this light spectrum, which is why we need optical aids for better sight at night. Thermal imaging technology allows the human eye to see this particular spectrum of light that we cannot normally see, making it much easier to see things that are giving off heat even in very low or no light conditions. (You can also watch this video for a more in-depth explanation.)