Ultraviolet rays are an invisible form of light. They lie just beyond the violet end of the visible spectrum. The sun is the major natural source of ultraviolet rays. Lightning, or any other electrical spark in the air, also emits ultraviolet rays. The rays can be produced artificially by passing an electric current through a gas or vapor, such as mercury vapor. The rays can cause sunburn. Overexposure can cause skin cancer. Ultraviolet rays also destroy harmful organisms and have other useful effects.