Even under very dark conditions, there is little bit of light present. Some of it can not be visible to naked eyes as it may be infrared. Infrared goggles are used to collect the available light and amplify it to see it in the dark. Every hot object including human body, emit heat in the form of IR light, using thermal capturing techniques, infrared goggles capture this infrared light making the object visible in dark.
Therefore, creation of light by radiating heat helps the soldiers to see in the dark.
"outside visible spectrum" would be the right way to define the power of the goggles provided to the soldiers to see at night. The correct option among all the options that are given in the question is the first option or option "A". I hope the answer comes to your great help.
