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)

Sources of light

• Light energy comes from different sources:– Natural– Artificial

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

Light of electric discharge

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

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

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

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

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)

Chemiluminescence

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

Bioluminescence

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

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)

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?

4) What is phosphorescence?

5) Why is chemiluminescence sometimes referred to as cool light?

6) What are two differences between LEDS and incandescent bulbs?