Intermediate Module: Creating Light Presentation: Fluorescent Lights

1. Intermediate Module: Creating Light Presentation: Fluorescent Lights

2. Fluorescent light is produced in tubes. Even the fluorescent lights that look like light bulbs actually have a tube inside of them.

3. Inside a fluorescent light tube is a small amount of a gas that won’t burn or react with anything else. The gas normally used is called argon. argon atoms

4. Inside a fluorescent light tube is a small amount of a gas that won’t burn or react with anything else. The gas normally used is called argon. • Also inside the tube is mercury, a metal that is a liquid at room temperature. Some of the mercury atoms evaporate, so before electricity is applied there is both liquid and gaseous mercury in the tube. liquidmercury mercuryatoms argonatoms

5. liquidmercury mercuryatoms argonatoms • Now let's see what happens when you make electricity go through the tube. • A stream of electrons is sent out from one end of the tube toward the other end.

6. liquidmercury mercuryatoms argonatoms • Now let's see what happens when you make electricity go through the tube. • A stream of electrons is sent out from one end of the tube toward the other end. • This flow of electrons goes through the mercury as well, heating it up and changing it from a liquid to a gas. mercuryatoms argonatoms

7. The electrons in the mercury atoms pick up energy from the stream of electrons going through the tube. When an electron gains energy in this way, it moves into an orbital with higher energy. mercuryatoms argonatoms highenergy lowenergy

8. After a very short time, the electron will drop back into an orbital with lower energy. When it does that, it emits a high-energy photon, in the ultraviolet range. mercuryatoms argonatoms highenergy ultraviolet photon lowenergy

9. All of this is happening with many, many mercury atoms at the same time. So many, many ultraviolet photons are emitted. mercuryatoms argonatoms

10. All of this is happening with many, many mercury atoms at the same time. So many, many ultraviolet photons are emitted. • If that were the end of the story, we wouldn't be able to see anything with fluorescent light bulbs, because we can't see ultraviolet photons. But there's another very important part of a fluorescent bulb. mercuryatoms argonatoms

11. highenergy lowenergy • The fluorescent tube is coated with a special substance called a phosphor. When ultraviolet photons strike the phosphor coating, their energy is absorbed by electrons in the atoms in that coating, and they move to a higher energy level.

12. highenergy lowenergy • The electrons are kicked to higher energy orbitals, but then something interesting happens. Instead of dropping all the way back to the low energy level they started at, they drop down to another energy level in between. So they emit photons with less energy.

13. The electrons are kicked to higher energy orbitals, but then something interesting happens. Instead of dropping all the way back to the low energy level they started at, they only drop down to another orbital in between. So they emit photons with less energy. highenergy These photons have a longer wavelength, which puts them somewhere in the visible light zone. lowenergy

14. Think About It • See if you can answer this question before you continue: • A particular phosphor emits photons of a particular wavelength. How do you think fluorescent bulbs produce white light, which is a combination of wavelengths?

15. Different phosphors emit different wavelengths of light. So fluorescent light bulbs usually use a few different phosphors, to produce combinations of wavelengths that we see as white.

16. Fluorescent bulbs are better than incandescent bulbs at turning electrical energy into light energy. • It takes 3 to 4 times as much electrical power to produce a certain amount of light for a incandescent bulb as to produce the same amount of light with a fluorescent bulb. 100 watts 22 watts

17. Let's review: • 1. A stream of electrons is sent out from one end of the tube to the other end. • 2. This causes electrons in the mercury atoms to emit ultraviolet light. • 3. The ultraviolet light strikes the phosphor coating of the tube, causing the electrons in the phosphor atoms to emit visible light. • 4. A combination of phosphors is used to produce a combination of wavelengths that we see as white light. • 5. Fluorescent bulbs are much better than incandescent bulbs at making light from electrical energy.

18. Think About It • Why do you think fluorescent bulbs need less electrical power than incandescent bulbs to produce the same amount of visible light?

19. Assessment • Do these steps to show what you have learned, and to help you to remember and be able to use what you have learned: • What is the role of mercury in fluorescent tubes? • Explain, in your own words, what a phosphor is. • Why is the light from an fluorescent bulb white? • Sketch this diagram on your paper, label the things it shows, and explain what it shows.