Reference;popularfishkeeping.co.uk January-February 2014
Air versus light Living plants depend on the energy from specific wavelengths of light, in order to be able to photosynthesis and thus grow. Simply placing a light over an aquarium will not guarantee that your plants will thrive – it needs to be of the right type. Water itself has what is known as a V value or refractive index. Every transparent item has its own refractive index and it is this index that dictates just how light passes through the medium concerned. Air is denoted by an index
of just 1.00. This means that light travels through air very well indeed, and is not changed very much in terms of concentration or color aberration, meaning that its brightness and color are not affected. In contrast, the refractive index of water is around 1.33 and as such, the light itself entering here will be concentrated into a smaller area and it becomes more subject to both color aberration and color change. In this type of situation, many PAR readings that are taken are inaccurate. A reading from a PAR meter taken through air will always be higher than if the same lamp is directed through water. When checking on the stated PAR level for your aquarium therefore, you should be clear that the figure advertised has indeed been taken through water and not through the air. It is also worth pointing out that if salt is added to an aquarium, even to create a brackish rather than a marine set-up, the refractive index
increases marginally, and this will still be sufficient to impact on the light and its passage through the water. In terms of cold water or tropical aquariums where live plants are not included, or if low light species of plants are chosen, then the PAR output of a lighting product does not matter too much. Fish are not reliant upon solar energy in the same way as reptiles and birds, and therefore a lower power light fitting can be safely used, without any risk of harm to their health. This light source, however, should be of a sufficient quality to show the fish themselves off in the very best light, so to speak