Most babies are able to begin sitting up when they reach the age of 4 months. However, some of them might need more time and start sitting independently when they become 7 months old. Until this time, the baby would definitely have learned how to roll and would be able to hold his head up too.
UV radiation, in the form of lasers, lamps, or a combination of these devices and topical medications that increase UV sensitivity, are sometimes used to treat patients with certain diseases who have not responded to other methods of therapy.
The advertised purpose of a Skylight or UV filter is to cut atmospheric haze by absorbing ultraviolet light. Since ultraviolet light can also reduce overall sharpness, absorbing UV rays will help sharpen your photos. A skylight filter will also reduce atmospheric blue by slightly warming a scene.