Many light sources (including the sun) give out infrared light, but your eyes do not detect it.
Digital cameras have a filter to stop them recording light you couldn't see, so the photos look close to what you would see if you were there.
You can remove this filter, and make your own infra-red passthru to only allow IR light into the camera.
Add an infrared light source, to create a night vision camera. You can record night animals without disturbing them. (such as glowworms, weta, or kiwi).
Note: most clothing is dyed to block visible light. Your camera can likely see right through, so beware when taking photos in public, best to not point this camera at people.
Open up your old digital camera.
Look for the infra-red light filter over the lens. Remove the filter. (If you want to save it, store it safely in a ziplock plastic bag with some cardboard so it doesn't bend.)
Put your camera back together again You camera now records infra-red light.
If you want to block visible light, take some already exposed film, and tape over the lens to block light.
Your photos will have a purple tinge from the film. You'll see all the objects that reflect infra-red light lit up (plants reflect lots).