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The general design of a simple fluorescent lamp consists of a sealed glass tube. The tube contains a small bit of mercury and a gas  kept under very low pressure. The tube also contains a phosphor powder, coated along the inside of the glass. The tube has two electrodes, one at each end, which are wired to an electrical circuit. The electrical circuit, which includes a starter and ballast, is hooked up to an alternating current (AC) supply. 

When the lamp is first turned on, the current travels through the path of least resistance, which is through the bypass circuit, and across the starter switch. This current then passes through the circuit heating up the filament in each electrode, which are located at both ends of the tube (these electrodes are simple filaments, like those found in incandescent light bulbs). This boils off electrons from the metal surface, sending them into the gas tube, ionizing the gas. The mercury vapor becomes "excited" and it generates radiant energy, mainly in the ultraviolet range. This energy causes the phosphor coating on the inside of the tube to fluoresce, converting the ultraviolet into visible light.