1. Light Sources

2. Luminescent materials Luminescent materials ~materials that glow when they get excited. ex. most metals, certain gases (neon) Artificial light sources depend on luminescent materials

3. Excited Atoms When certain atoms/molecules take in energy their electrons jump to a higher unstable energy level. When the electrons return to normal energy level, they release energy as a photon. A photon is a packet of energy. This is the fundamental unit of light.

4. Incandescent (normal) light bulb Electricity flows along the wire The (center wire) filament has a high resistance, so it gets hot. So hot it begins to glow (it is normally made of tungsten) Gases inside get excited and begin to glow as well It burns out when the filament breaks from too much friction. These waste a lot of energy as heat but give off a pleasing light. They are also cheap to produce.

5. Fluorescence Fluorescence- some type of EM radiation (commonly UV) is absorbed by a substance and visible light is reemitted immediately. Fluorescent light bulbs do this. Ultraviolet light hits the phosphorus coating (white powder), it is absorbed and reemitted as visible light. Several other substances will fluoresce. Which is why certain substances glow in a black light, which is ultraviolet light.

6. Fluorescent light bulbs There is a low pressure gas in the tube (no wire connecting the ends) You get “sparks” shooting through the tube which excites the gases in the tube. These gases give off UV light. There is a phosphorus coating on the tube that fluoresces, absorbs the UV light and gives off a white light. These don’t get as hot, or waste as much electricity but are more expensive and the light is not as pleasing.

7. LED light bulbs Light Emitting Diode A Diode is two semiconducting materials sandwiched together to create a cathode and an anode (positive and negative). Electric current (electrons) flow through and there are “holes” or places where the electrons can drop to lower energy levels. When they drop the release photons of light. Until recently, these were expensive to make. They consume less power and last longer than the other types of light bulbs

8. Phosphorescent substances When some substances absorb energy they jump to a metastable state. (slightly stable) They slowly reemit the energy (over a few hours). So you can “charge” the substance in light, they have it reemitted later on. These are called “glow in the dark” Commonly used on watch faces and toys