The Production of Light – Optics Lesson

- Incandescence- Light-Emitting Diode (LED)- Fluorescent lights- Compact Fluorescent lights (CFLs)- Electric Discharge- Phosphorescence- Fluorescence- Chemiluminescence- Bioluminescence

Recall• We see things because light enters our eyes

from all objects• Some are luminous like the Sun and can

produce their own light• Some are non-luminous like us and only reflect

light

Light from Incandescence• Any object, as it gets hotter and hotter,

will eventually produce light (observe the molten metal on the right).

• As the object gets hotter, the colours of light produced change from red, to orange, to yellow, to white, and then to bluish-white.

• This process of producing light as a result of high temperature is called incandescence.

• Light from a burning candle is an example of incandescence.

Light from Incandescence• Incandescence also occurs in an incandescent light bulb.

A thin wire filament, usually made of tungsten, glows as electricity passes through it. The filament becomes so hot that it gives off visible light.

• Only 5 % to 10 % of the electricity going through the filament is actually converted into visible light. The rest of the energy is converted into infrared light (heat).

• For this reason, incandescent bulbs are very inefficient light sources and will become extremely hot!

How Incandescent bulbs workHow they are made

Light from a Light-Emitting Diode (LED)

• A LED is an electronic device that allows an electric current to flow in only one direction.

• It differs from an incandescent light as it does not require a filament; it does not produce much heat as a by-product; and it is more energy efficient.

• LEDs are now used in Christmas lights illuminated signs, and traffic lights.

Fluorescent lights• A fluorescent light makes use of both

electric discharge and fluorescence.• A fluorescent light tube is filled with very

low-pressure mercury vapour. The inner surface of the tube is also coated with a fluorescent material.

• When a fluorescent light is turned on, the electric current causes the mercury atoms to emit ultraviolet light. This ultraviolet light then strikes the fluorescent inner surface of the tube, resulting in the production of visible light

Compact Fluorescent lights (CFLs)

• A fluorescent light is much more efficient and therefore can provide the same light output as an equivalent incandescent bulb but produces much less heat and uses far less electricity

Light from Electric Discharge• Is produced by an electric current passing

through a gas. • The electricity causes the gas to glow.• Examples – Lightning, Neon Signs

Light from Phosphorescence• Objects that “glow-in-the-dark” such as wristwatches

and clocks are coated with phosphors which give off light through a process called phosphorescence.

• Phosphors absorb light energy then release it as visible light over time from seconds up to days, depending on the material.

Light from Fluorescence• Fluorescence occurs when an object absorbs ultraviolet

light and immediately releases the energy as visible light.

• Highlighter pens are an example. The ink in these pens contains a fluorescent dye that causes the ink to glow in the presence of the ultraviolet part of normal daylight.

Light from Chemiluminescence• Chemiluminescence is the production of light as a

direct by-product of a chemical reaction.• Light sticks operate by causing two chemicals to mix.• Bending the light stick in the middle causes a small

glass vial to break, allowing two chemicals to mix producing visible light.

Light from Bioluminescence• When chemiluminescence occurs in

living organisms, scientists call it bioluminescence.

• Bioluminescence occurs in a wide variety of organisms, including certain bacteria, fungi, marine invertebrates, fish, and the well known examples of glow-worms and fireflies

Bioluminescence