Transcript 18.5 Sources of Light - Henry County Schools

18.5 Sources of Light
Objects are
invisible in the dark
because no light is
available to reflect
off them. But some
things, such as
flashlights and
fireflies, and the
light sources in this
hallway, produce
their own light.
18.5 Sources of Light
What are the six common sources of light?
Common light sources include
incandescent, fluorescent, laser, neon,
tungsten-halogen, and sodium-vapor bulbs.
18.5 Sources of Light
Objects that give off their own light are luminous.
The sun is luminous, as are all light sources.
18.5 Sources of Light
Incandescent Light
How does an incandescent light source
generate light?
The color of any object depends on what the
object is made of and on the color of light
that strikes the object.
18.5 Sources of Light
Incandescent Light
The light produced when an object gets hot
enough to glow is incandescent.
The filaments in incandescent light bulbs are
made of a substance called tungsten.
Incandescent bulbs give off most of their energy
as heat, not light.
18.5 Sources of Light
Incandescent Light
An incandescent bulb
contains a filament. As
electrons flow through
the filament, the
filament gets hot and
emits light.
18.5 Sources of Light
Fluorescent Light
How does a fluorescent light source
generate light?
Fluorescent light bulbs emit light by causing
a phosphor to steadily emit photons.
18.5 Sources of Light
Fluorescent Light
In a process called fluorescence, a material
absorbs light at one wavelength and then emits
light at a longer wavelength.
A phosphor is a solid material that can emit light
by fluorescence.
A fluorescent bulb is a glass tube, containing
mercury vapor, that is coated with phosphors.
18.5 Sources of Light
Fluorescent Light
The electrodes in a
fluorescent bulb emit
electrons that cause
the mercury atoms to
emit ultraviolet rays.
These rays cause the
phosphor coating to
emit light.
Glass tube filled
with mercury
vapor
Electrode
Electrical
contacts
18.5 Sources of Light
Fluorescent Light
Office buildings and schools use mostly
fluorescent lights.
Fluorescent tubes do not get as hot as
incandescent bulbs because they emit most of
their energy as light. This means that they use
energy very efficiently.
18.5 Sources of Light
Laser Light
How does a laser light source generate
light?
Laser light is emitted when excited atoms of
a solid, liquid, or gas emit photons.
18.5 Sources of Light
Laser Light
Light in which waves have the same wavelength,
and the crests and troughs are lined up, is
coherent light.
• A laser is a device that generates a beam of
coherent light.
• A beam of coherent light doesn’t spread out
significantly from its source, so the light has a
relatively constant intensity.
• The energy it carries may be focused on a small
area.
18.5 Sources of Light
Laser Light
Lasers have many practical applications.
• Lasers can cut through metals and make computer
chips.
• Surgeons use lasers to cut or repair damaged
tissue.
• Lasers carry information through optical fibers.
• Laser light is used to measure distances precisely.
18.5 Sources of Light
Laser Light
The waves in laser light all have the same
wavelength and direction of travel, and their
peaks coincide.
Neon atom
Tube wall
Mirror
Laser beam
Mirror
Helium atom
Coherent waves
18.5 Sources of Light
Neon Light
How does a neon light source generate
light?
Neon lights emit light when electrons move
through a gas or a mixture of gases inside
glass tubing.
18.5 Sources of Light
Neon Light
Many lights called neon lights contain gases other
than neon, including helium, argon, and krypton.
• Helium gas gives off a pink light.
• A mixture of argon gas and mercury vapor produces
greenish-blue light.
• Krypton gas produces a pale violet light.
• Pure neon emits red light.
18.5 Sources of Light
Neon Light
• Each kind of gas emits photons of different
energies, and therefore different colors.
• The different photons emitted combine to give
each glowing gas a distinctive color.
• The color of glass used to make the tube can
also affect the color of the light.
18.5 Sources of Light
Sodium-Vapor Light
How does a sodium vapor light source
generate light?
As electric current passes through a
sodium-vapor bulb, it ionizes the gas
mixture. The mixture warms up and the heat
causes the sodium to change from a solid
into a gas.
18.5 Sources of Light
Sodium-Vapor Light
Sodium-vapor lights contain a small amount of
solid sodium, in a mixture of neon and argon
gases.
The current of electrons knocks electrons in
sodium to higher energy levels. When the
electrons move back to lower energy levels, the
sodium atoms emit light.
18.5 Sources of Light
Sodium-Vapor Light
The yellow color of sodium-vapor light makes
objects look different than they look in sunlight.
18.5 Sources of Light
Tungsten-Halogen Light
How does a tungsten halogen light source
generate light?
Inside a tungsten-halogen bulb, electrons
flow through a tungsten filament. The
filament gets hot and emits light.
18.5 Sources of Light
Tungsten-Halogen Light
Tungsten-halogen light is produced in much the
same way as incandescent light.
A tungsten-halogen bulb has a small amount of a
halogen gas, such as iodine, bromine, or fluorine.
The halogen gas reduces wear on the filament,
so tungsten-halogen bulbs last longer than
incandescent bulbs.
18.5 Sources of Light
Assessment Questions
1. The light produced when an object becomes hot
enough to glow is
a.
b.
c.
d.
incandescent.
fluorescent.
phosphorescent.
coherent.
18.5 Sources of Light
Assessment Questions
1. The light produced when an object becomes hot
enough to glow is
a.
b.
c.
d.
incandescent.
fluorescent.
phosphorescent.
coherent.
ANS: A
18.5 Sources of Light
Assessment Questions
2. The most efficient source of lighting rooms of a
building is
a.
b.
c.
d.
incandescent light.
flourescent light.
sodium-vapor light.
tungsten-halogen light.
18.5 Sources of Light
Assessment Questions
2. The most efficient source of lighting rooms of a
building is
a.
b.
c.
d.
incandescent light.
flourescent light.
sodium-vapor light.
tungsten-halogen light.
ANS: B
18.5 Sources of Light
Assessment Questions
1. Neon lights emit coherent light, in which the
waves all have the same wavelength and wave
crests and troughs are lined up.
True
False
18.5 Sources of Light
Assessment Questions
1. Neon lights emit coherent light, in which the
waves all have the same wavelength and wave
crests and troughs are lined up.
True
False
ANS:
F, Lasers