A refrigerator comprising: a cabinet forming a storage space, a main control part which controls the operation of the refrigerator, a lighting unit provided at the storage space to illuminate the storage space; a main door to open and close the storage space and having an opening part; and a sub door to open and close the opening part and including a panel assembly through which light from the lighting unit is transmitted to an outside of the refrigerator, wherein the panel assembly comprises a front panel forming a front surface of the sub door, the front panel includes a see-through area through which light from the lighting unit is transmitted when the lighting unit is turned on, and a bezel area on which a bezel is formed and provided outside the see through area, wherein there is a detection device being attached to the rear surface of the front panel and checking whether a user stands in front of the refrigerator, characterised in that the refrigerator further comprises a knock detection device to detect a user's knocking operation applied on the front panel, wherein the lighting unit is turned on/off when the knock detection device detects the user's knocking operation applied on the front panel while the sub door is closed, wherein the front panel is being formed of a half mirror reflecting a part of the light and transmitting a part of the light to be selectively transparent, and wherein the see-through area of the front panel allows the user to view an inside of the refrigerator when the lighting unit is turned on while the sub door is closed, and forms a mirror surface while the lighting unit is turned off. a cabinet forming a storage space, a main control part which controls the operation of the refrigerator, a lighting unit provided at the storage space to illuminate the storage space; a main door to open and close the storage space and having an opening part; and a sub door to open and close the opening part and including a panel assembly through which light from the lighting unit is transmitted to an outside of the refrigerator, wherein the panel assembly comprises a front panel forming a front surface of the sub door, the front panel includes a see-through area through which light from the lighting unit is transmitted when the lighting unit is turned on, and a bezel area on which a bezel is formed and provided outside the see through area, wherein there is a detection device being attached to the rear surface of the front panel and checking whether a user stands in front of the refrigerator, characterised in that the refrigerator further comprises a knock detection device to detect a user's knocking operation applied on the front panel, wherein the lighting unit is turned on/off when the knock detection device detects the user's knocking operation applied on the front panel while the sub door is closed, wherein the front panel is being formed of a half mirror reflecting a part of the light and transmitting a part of the light to be selectively transparent, and wherein the see-through area of the front panel allows the user to view an inside of the refrigerator when the lighting unit is turned on while the sub door is closed, and forms a mirror surface while the lighting unit is turned off.