The optical properties of two water supply samples Kathmandu Upatyaka Khanepani Limited (KUKL) and Kathmandu Valley Water Supply Improvement Project (KVWSIP) in the Kupondole Area, Lalitpur, Nepal show that the transmittance of light is higher for KVWSIP sample water than for KUKL. A large amount of work has been done to test the purities of KUKL and KVWSIP water supplies (chemically), but even their optical properties have not been studied. This method has no hazardous side effects or requires chemical method testing. In comparing the optical properties, the authors recommended that the public take KVWSIP water for good health and wealth rather than KUKL if they have an option.This is because KUKL is more contaminated and has more total dissolved solids (TDS) particles, while KVWSIP has fewer TDS particles. Therefore, if people take KVWSIP water in their daily life, they obtain relief from different types of water-related diseases.