Usually we are not able to see infrared rays with our naked eyes. But we could see it when we watch it through a digital camera (or mobile camera). Why it happens?
This isn't really an electrical engineering question, but the short answer is that your eye has limitations. All light is a subclass of a larger form of energy called electromagnetic radiation.
You can't see EM radiation with a wavelength shorter than around 730 nanometers, or what our brains perceive as red. However, your camera doesn't have the same limits, and will pick up the infrared light and try to display it just like any other light.
Your camera is based upon Si, which has a bandgap of 1.12 eV which means that for light that is ~ 1.1 u and shorter the camera can possibly detect it. This range of wavelengths is in the part of the spectrum called Near-IR. You eyes cannot see wavelengths LONGER than 720 nm. Near IR transmitters used in TV remotes etc. typically are 850 nm.