1. 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.

2. What are the six common sources of light?
Common light sources include incandescent, fluorescent, laser, neon, tungsten-halogen, and sodium-vapor bulbs.

3. Objects that give off their own light are luminous
Objects that give off their own light are luminous. The sun is luminous, as are all light sources.

4. 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.

5. 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.

6. Incandescent Light
An incandescent bulb contains a filament. As electrons flow through the filament, the filament gets hot and emits light.

7. Fluorescent Light
How does a fluorescent light source generate light?
Fluorescent light bulbs emit light by causing a phosphor to steadily emit photons.

8. 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.

9. Glass tube filled with mercury vapor
Fluorescent Light
Glass tube filled with mercury vapor
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.
Electrode
Electrical contacts

10. 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.

11. 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.

12. 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.

13. Lasers have many practical applications.
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.

14. Laser Light
The waves in laser light all have the same wavelength and direction of travel, and their peaks coincide.
Mirror
Helium atom
Coherent waves
Laser beam
Tube wall
Neon atom

15. 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.

16. 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.

17. 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. 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.

19. 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.

20. Sodium-Vapor Light
The yellow color of sodium-vapor light makes objects look different than they look in sunlight.

21. 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.

22. 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.

23. The light produced when an object becomes hot enough to glow is
Assessment Questions
The light produced when an object becomes hot enough to glow is
incandescent.
fluorescent.
phosphorescent.
coherent.

24. The light produced when an object becomes hot enough to glow is
Assessment Questions
The light produced when an object becomes hot enough to glow is
incandescent.
fluorescent.
phosphorescent.
coherent. ANS: A

25. The most efficient source of lighting rooms of a building is
Assessment Questions
The most efficient source of lighting rooms of a building is
incandescent light.
flourescent light.
sodium-vapor light.
tungsten-halogen light.

26. The most efficient source of lighting rooms of a building is
Assessment Questions
The most efficient source of lighting rooms of a building is
incandescent light.
flourescent light.
sodium-vapor light.
tungsten-halogen light. ANS: B

27. Assessment Questions
Neon lights emit coherent light, in which the waves all have the same wavelength and wave crests and troughs are lined up. True False

28. Assessment Questions
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