A viewing instrument has an optical module having a first light pathway for conveying visible light reflected from an object to an eye of a user viewing the object; a thermal radiation detecting module having a thermal radiation detector adapted to detect thermal radiation emitted from the object being viewed to thereby generate one or more signals; an aiming module having at least one visible light-emitting member electrically connected with the thermal radiation detecting module, wherein the aiming module is adapted to receive the one or more generated signals from the thermal radiation detecting module, and in response to the received one or more signals, project a visible mark along the first light pathway to the eye of the user. The invention also provides a toy gun having the viewing instrument as described above.