How Do Fluorscent Light Bulbs Work

This Exemplar will teach the following concept(s) from the ACS Examinations Institute General Chemistry ACCM:

I. D. 1. a. The interaction of photons with molecules and atoms provides information about energy levels.

IX. E. 3. a. Spectral data can be used to demonstrate quantized energy levels in atoms in the gas phase.

 

How CFLs Work

The light bulb, a common household appliance, is frequently overlooked. Most people simply screw it in and forget about it, but how do they work? Compact fluorescent lamps, or CFLs, are more interesting than most might think and incorporate many areas of chemistry. However, to truly understand CFLs work, one must know the electronic structure of atoms and how these electrons are involved in the emission of light. 

 

Compact fluorescent lamps or CFLs are fairly simple in their working. A glass bulb is filled with mercury vapor and an inert gas (neon, argon, xenon, or krypton), and has an inner coating called a phosphor.¹ Electrodes add electrons into the bulb, exciting the mercury vapor. When mercury then returns to ground state to become stable again, it will emit a photon of energy to transition back to its normal state.² The photons emitted have wavelengths that are not on the visible spectrum, so the inner coating, or phosphor, fixes this. The phosphor layer works on the same principle as the mercury vapor, as it will absorb the photon of energy, exciting its electrons, and after returning to its ground state, release a different photon of lower energy that has a wavelength on the visible spectrum. The phosphor layer accomplishes this through the properties of its composition, a blend of metallic metals such as: copper, zinc, aluminum, or rare earth metals.³ Depending on the composition, the wavelength released by the phosphor layer can vary giving different visible colors. 

 

Energy Level

The movement of electrons throughout an atom's subshells is vital to the function of fluorescent light bulbs. The subshells of an atom are filled from lowest to highest energy level. This is because the energy required for an atom to begin filling a new shell is significantly higher than the attractive force of the nucleus. In a fluorescent light bulb, an electric current is used to introduce energy to the mercury vapor, exciting it. This excited state causes an electron to jump to a higher energy level, increasing mercury's energy causing mercury to become less stable. In order to reduce its energy level back to its previous and more stable state, its ground state, mercury emits a photon of energy corresponding with the amount lost from returning the electron to its previous energy level.

 

FIG 1: Basic representation of absorption to a upper state, and emission back down to ground state.⁴

FIG 2:Mercury absorbs energy at 4.87 eV to populate its excited state. When returning to ground state mercury releases a photon with a wavelength of 253.7 nm.²

 

Photon Emissions and Phosphor Layers

The released photon of energy by mercury has an ultraviolet wavelength around 254 nm, the problem is, this wavelength is not on the visible light spectrum.² To fix this an inner coating called the phosphor layer is utilized. This layer is composed of blends of metallic metals such as copper, zinc, silicon, and or rare earth metals.³  This layer works on the same electron structure aforementioned. The photon released from the mercury excites the phosphor layer causing the electrons of the metals to jump to a higher energy level. Wanting to return to their ground state and more stable form, the metals release a photon of energy to release the excess energy. This new photon of energy now has a wavelength that is on the visible spectrum.

Exercise \(\PageIndex{1}\)

The mercury vapor releases 4.87 electron volts per photon. After passing through the phosphor layer green light is emitted with a wavelength of 543 nm. How much energy did the phosphor layer absorb?

Answer

c (speed of light)/ (wavelength) = v (frequency)

2.998e8m/s /543e-9m = v = 5.52e14 s-1

 

E (energy of a proton) = v (frequency) * h (Planck's constant)

E = 5.52e14 s-1*6.63e-34 J*s

E = 3.66e-19 J = 2.28 eV

 

1 Joule = 6.242e18 electron Volts

4.87 eV - 2.28 eV = 2.58 eV

Different compositions of the phosphor layer create different colors of light that are emitted, resulting in warm light, and white light for example. In modern fluorescent light bulbs, a triphosphoric blend is typically used to create three colors: red, blue, and green.3 These three colors can be mixed to create different colors of light emitted from the light bulb. Commonly used, Y₂O₃, activated by Eu³⁺, emits a red photon of light with a wavelength around 625 nm, CeMgAl₁₁O₁₉, activated by Tb³⁺, emits a green photon of light with a wavelength around 543 nm, and BaMgAl₁₀O₁₇, activated by Eu²⁺, emits a blue photon of light with a wavelength around 450 nm.³

Exercise \(\PageIndex{2}\)

After absorbing a photon of energy of 4.87 eV released by mercury, the phosphor layer absorbs 2.899 eV.  Find the wavelength of the photons emitted by a phosphor layer. What color are the photons?

Answer

4.87 eV-2.899 eV  = 1.97 eV 

6.242e18 electron Volts=1 Joule

= 3.15e-19 J

 

1.60218e-19 Joule = 1 electron Volts

 

3.15e-19/6.626e-34=4.76e14 s^-1

E (energy of a proton) / h (Planck's constant)= v (frequency)

 

3.00e8/4.76e14=629 nm 

c (speed of light) / v (frequency) = λ (wavelength)

Orange Light

 

 References

1. The Fluorescent Lamp - How it Works & History. https://edisontechcenter.orghttps://edisontechcenter.org/Fluorescent.html#howitworks. (accessed November 10, 2022).

 

2. Energy-Saving Lamps & Health. https://ec.europa.eu/health/scientif...and%20185%20nm. (accessed November 10, 2022).

 

3. Srivasta, Alok M.; Sommerer, Timothy J.. Fluorescent Lamp Phosphors: Energy Saving, Long Life, High Efficiency Fluorescent Lamp Products. The Electrochemical Society 1998, 7 (2), 17-32. (accessed November 10, 2022).

4. Energy Level Diagram - Different Energy Shells Around the Nucleus https://byjus.com/chemistry/energy-level-diagram/. (accessed November 10, 2022).