The simple flashlight consists of batteries, a bulb and a switch in a tube. While the basic version is still around, there are much more complex flashlights available today. The standard incandescent bulb flashlight that most people are familiar with can be found in almost any retail outlet. There are also the cheaply made dollar-store flashlights, as well as disposable and rechargeable models.
The basic flashlight uses one or two batteries inside a plastic or metal tube that will send a current from the batteries through the filament of a bulb. There will be some sort of switching mechanism on the body of the flashlight that allows current to flow or stop at the user's will. The electrical current creates heat which is then seen as light waves. The intensity of a flashlight depends on the bulb, how much current is running through it and the ability of the reflector behind the bulb to focus light on an object.
Light-emitting diode models incorporate special bulbs and micro-circuitry that can vary greatly in cost and output. Newer LED lamps emit light in its base form as photons. They do this by boosting electrons to higher energy states and then dropping them back to lower states, which releases photons. LED circuitry can get very hot and requires the higher current output of CR123 batteries to operate.
High-intensity discharge flashlights produce light by arcing electricity over a gap between two tungsten electrodes inside an enclosed tube, but these mostly used mostly by military personnel and public safety officials. They are much more efficient than incandescent flashlights and LED flashlights, because more of the energy generated is turned into visible light instead of being wasted as heat. This type of flashlight can vary in cost and quality, with some units costing over $5,000!