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 provided inside the refrigerator is turned on, and functions as a mirror while said lighting unit 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 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 provided inside the refrigerator is turned on, and functions as a mirror while said lighting unit 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 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 . <EOS>