Satco S6506 4100K 40-Watt 4 Pin T9 Circline Lamp, Cool White;

The glass tube of a fluorescent lamp is filled with a gas containing argon and mercury vapor. The interior wall of the tube is coated with a paint known as phosphor. At each end of the tube, electrodes send electrons through the gas causing it to emit ultraviolet light as the ultraviolet light passes through the phosphor coating, it is converted into a longer frequency to create visible light. The glass tube of the bulb prevents the harmful UV rays from escaping. Fluorescent lamps are more efficient than incandescent lamps because less energy is converted to heat. Instead, most of the energy used creates visible light.