A refrigerator comprising: a cabinet configured to form a storage space; a door configured to open and close the cabinet, and having an opening which is in communication with the storage space; a panel assembly provided at the opening, having a structure like a half glass which is see-through while a lighting unit (49, 85) provided inside the refrigerator is turned on, and functions as a mirror while said lighting unit (49, 85) is not turned on, allowing to selectively see through to the inside of the refrigerator, the panel assembly comprising a front panel which forms an exterior of a front surface thereof, at least one insulation panel which is formed to be spaced apart from a rear surface of the front panel, and a spacer bar provided between the front panel and the insulation panel and a spacer bar between each two insulation panels of the plurality of insulation panels, in case there is more than one insulation panel; and said lighting unit (49, 85) provided inside the refrigerator, and turned on or off by a signal of a detection device while the door is closed, such that the storage space of the cabinet is seen from the outside through the panel assembly, wherein the detection device is in contact with a rear surface of the front panel of the panel assembly; characterized in that the detection device is a knock detection device disposed to be in close contact with the panel assembly, and configured to detect a user's knocking operation on the panel assembly, and in that a bezel which does not transmit light is formed at an edge of the rear surface of the front panel, and the knock detection device is disposed at an area of the bezel. a cabinet configured to form a storage space; a door configured to open and close the cabinet, and having an opening which is in communication with the storage space; a panel assembly provided at the opening, having a structure like a half glass which is see-through while a lighting unit (49, 85) provided inside the refrigerator is turned on, and functions as a mirror while said lighting unit (49, 85) is not turned on, allowing to selectively see through to the inside of the refrigerator, the panel assembly comprising a front panel which forms an exterior of a front surface thereof, at least one insulation panel which is formed to be spaced apart from a rear surface of the front panel, and a spacer bar provided between the front panel and the insulation panel and a spacer bar between each two insulation panels of the plurality of insulation panels, in case there is more than one insulation panel; and said lighting unit (49, 85) provided inside the refrigerator, and turned on or off by a signal of a detection device while the door is closed, such that the storage space of the cabinet is seen from the outside through the panel assembly, wherein the detection device is in contact with a rear surface of the front panel of the panel assembly; characterized in that the detection device is a knock detection device disposed to be in close contact with the panel assembly, and configured to detect a user's knocking operation on the panel assembly, and in that a bezel which does not transmit light is formed at an edge of the rear surface of the front panel, and the knock detection device is disposed at an area of the bezel.