To overcome the disadvantages introduced by using UV sensors to detect the intensity of UV light in water purification apparatuses, a novel detection apparatus to “visualize” the quality of water in the form of visible light, instead of digitizing the intensity of UV light includes s a first detection window, coated with a first material for converting a received first ultraviolet light into a first visible light. The first ultraviolet light is emitted from an ultraviolet light source and traverses the liquid, and the detection apparatus further mixes the first visible light with second visible light to generate a third visible light. The different color of the third visible light can represent the different quality of the water.