1. Types of Light Emissions
Luminous: an object that emits/produces its own light (eg. sun)
Non-luminous: an object that do not produce its own light and is visible due to reflected light (eg. a tree)

2. Sources of light Light energy comes from different sources: Natural
Artificial

3. Incandescence
light emitted from a material (tungsten metal) because of the high temperature of the material
About 5% of the electrical energy used in an incandescent bulb becomes light, the rest is lost as heat

4. Light of electric discharge
process of producing light by passing an electric current through a gas (eg. a neon sign is red-orange; lightning)

5. Light from fluorescence
Luminescence
The emission of light by a material or an object that has not been heated (eg. fluorescence)
Light from fluorescence
Process of emitting light while receiving energy from another source

6. How does a fluorescent bulb work?
It is an electric discharge tube
The bulb contains mercury vapour along with an inert gas (Argon).
The inside of the bulb is coated with a powdery substance called phosphor
As electrons travel through the gas, from one electrode to the other, they collide with atoms of mercury and excite these atoms

7. How does a fluorescent bulb work?
The excited mercury atoms release their excess energy in the form of ultraviolet (UV) light (humans can’t see this)
The energy of the UV light is absorbed by the phosphor, which emits visible light
These light bulbs are 20% efficient, meaning 20% of the energy it uses is converted into light

8. Phosphorescence
Light that is emitted due to exposure of the source to white light, and that continues to be emitted for some time in the absence of the white light (eg. glow in the dark objects)

9. Chemiluminescence
Light that is produced by a chemical reaction without a rise in temperature (eg. glow stick)

10. Bioluminescence
Light that is produced by a biochemical reaction in a living organism (eg. firefly)

11. Light emitting diode (LED)
Triboluminescence
Production of light from friction as result of scratching, crushing, or rubbing certain crystals
Light emitting diode (LED)
Light produced as a result of an electric current flowing in semiconductors; no filaments and does not produce much heat (eg. Christmas lights)

12. Review questions
1) What do all incandescent materials have in common to cause them to emit light?
2) What percentage of electrical energy used in an incandescent light bulb is converted to light?
3) How is the ultraviolet radiation produced in a fluorescent light transformed into visible light?

13. 4) What is phosphorescence
4) What is phosphorescence? 5) Why is chemiluminescence sometimes referred to as cool light? 6) What are two differences between LEDS and incandescent bulbs?