Lecture 30: Light Emission Processes and Technologies

Atomic Energy Levels and Electron Transitions

• Electrons exist around the nucleus of an atom in wave-like orbitals, which are characterized by specific amounts of energy.
• Each atom possesses a unique set of energy levels that its electrons can inhabit. These levels are discrete, meaning electrons can only occupy these set quantities and cannot exist between levels.
• The state with the lowest possible energy for an electron is known as the ground state, while any level above this is referred to as an excited state.
• Analogy: The potential energy levels of a human in a building where one can only move between floors by jumping. In this scenario, potential energy is discrete because you can only exist on one floor or another, with no possibility of remaining between floors (as there are no stairs or elevators).
• Electrons transition between energy levels by gaining or losing specific amounts of energy:
  • Excitation: An electron jumps to a higher level when it gains energy from light waves or through collisions with other electrons or subatomic particles.
  • Decay: An electron in an excited state will eventually decay back to its ground state. During this decay, it emits a photon of light.

The Physics of Light Emission

• The light emitted during electron decay has an energy directly proportional to its frequency.
• The energy of the emitted light is exactly equal to the difference in energy between the two levels involved in the transition.
• Planck’s Equation: The frequency ($f$) of the light is calculated using the formula:   $E = hf$
• Planck’s Constant ($h$): A fundamental constant used in quantum mechanics, equal to:   $h = 6.6 \times 10^{-34}\,\text{Joule-seconds}$
• Each element has a unique emission spectrum because of its specific energy-level configuration. Electrons can decay between any two levels, producing multiple possible frequencies:
  • For an atom with four energy levels (one ground state and three excited states), there are six possible transitions, resulting in six distinct frequencies of light.

Emission and Absorption Spectra

• Emission Spectrum: Displays the specific frequencies of light emitted by a material as its electrons decay. This acts as a chemical fingerprint.
  • Scientists determine the composition of stars by examining their emission frequencies and comparing them to known spectra of gases like Hydrogen and Helium.
  • Gas discharge lamps produce spectral lines that can be analyzed using a diffraction grating.
• Absorption Spectrum: Displays the wavelengths of light absorbed by a material.
  • When white light (which contains all visible frequencies) passes through a sample, the sample absorbs specific frequencies corresponding to its unique energy-level diagram.
  • These absorbed frequencies appear as black lines within the continuous rainbow of the white light spectrum.

Gas Discharge and Specialty Lighting

• Neon Lights: Traditionally utilized in storefront signs before the ubiquity of LEDs. Neon, a noble gas, specifically emits an orange color, though other gases can be used to produce different colors.
• Sodium Vapor Lights: Recognizable by their unique yellow hue, these were commonly used for highway lighting due to their high intensity and efficiency for night driving. Many are now being replaced by LEDs for a more pleasant white light.
• Diffraction Gratings: These tools are used to measure and visualize the discrete spectral lines emitted by gas discharge lamps.

Incandescence and Heat Transfer

• Incandescence: A process where light is emitted due to the temperature of an object. Previously discussed in the context of heat transfer (Lecture 16).
• Every object emits electromagnetic radiation based on its temperature:
  • Cooler objects (humans, plants, animals): Emit radiation in the infrared portion of the spectrum.
  • Hotter objects: Emit higher frequencies. At sufficiently high temperatures, they emit visible light.
• Color as a temperature indicator: Red or orange incandescent objects are cooler than those glowing white or blue.
• Incandescent Light Bulbs: Utilize a thin filament heated by an electric current. The heat causes the filament to emit multiple overlapping wavelengths that combine to form white light.
• Inefficiency: Incandescence is highly inefficient as most electrical energy is wasted as heat rather than light.

Comparative Efficiency: Incandescent vs. LED

• Hand-Crank Generator Demonstration:
  • Lighting an incandescent bulb requires significant physical effort (energy) and produces only dim light.
  • Lighting an LED bulb is very easy and requires much less energy.
• AC vs. DC in LEDs:
  • During the demonstration, the LED only illuminates for half of a cycle when powered by an AC generator because LEDs act as diodes.
  • In household applications, this is resolved by converting Alternating Current (AC) to Direct Current (DC).
• Daily Examples of Incandescence: Electric stoves and toasters use resistive heating elements. They glow red, then orange, and potentially yellow as they heat up. When turned off, they continue to emit infrared light while cooling to room temperature.

Fluorescence and Biological Imaging

• Fluorescence: A process where a material absorbs high-frequency light and re-emits it at a lower frequency. A small amount of heat is generated during the process, ensuring the conservation of energy.
• Medical Applications: Fluorescent dyes are attached to specific cells or biological structures.
  • High-frequency exciting light is filtered out, leaving only the lower-frequency light emitted by the dye visible.
  • This allows for the high-resolution imaging of biological processes and the measurement of cancer or bacteria volumes.
• Fluorescence in Nature: Observed in certain fish, animals, and minerals (when exposed to UV light). Chlorophyll in plants is a naturally occurring fluorescent molecule.

Phosphorescence and Phosphors

• Phosphorescence: Similar to fluorescence, but the re-emission of light is delayed. Atoms remain in an excited state longer, potentially for seconds, minutes, or hours after the source of excitation is removed.
• Applications: used in "glow-in-the-dark" paints that charge during the day under sunlight and emit light slowly at night.
• Phosphors: Chemicals used to create white light. By mixing different phosphorescent or fluorescent chemicals, engineers can combine Red, Green, and Blue (RGB) light to produce white light for home environments.

Fluorescent Lighting Engineering

• Mechanism: A glass tube contains mercury vapor, which is converted into plasma by a high voltage.
• UV Emission: Mercury plasma emits high-energy Ultraviolet (UV) light. While useful for sterilization, this ionizing radiation is dangerous for home use.
• Phosphor Coating: The interior of the tube is coated with phosphors that absorb UV light and re-emit it as visible light.
• Color Temperature: Controlled by the chemical makeup of the phosphor coating.
  • Cool lights: Emit more blue wavelengths.
  • Warm lights: Emit more yellow wavelengths.
• Spectrum: A fluorescent bulb's spectrum shows discrete lines from the mercury vapor emission blurred together by the broader emission spectrum of the phosphors.

Light-Emitting Diodes (LEDs)

• Structure: LEDs consist of two layers of semiconducting material stacked together.
• Mechanism: When voltage is applied in the correct direction (the "one-way valve" property of a diode), electrons flow from low to high voltage while "holes" (the absence of electrons) flow from high to low voltage. They recombine at the junction to emit light.
• Monochromaticity: A single LED generates only a specific frequency determined by the semiconductor material used. These range from infrared to UV.
• Generating White LED Light:
  1. RGB Packaging: Combining individual Red, Green, and Blue LEDs in one package (used in tunable bulbs).
  2. UV/Phosphor Method: A UV LED is placed in an epoxy housing coated with a phosphor that converts the light to white.
• Photovoltaic Panels: These use the reverse process of an LED. Absorbed light releases electron-hole pairs in a semiconductor to generate an electric current without fossil fuels.

Lasers: Light Amplification by Stimulated Emission of Radiation

• The Laser Acronym: Light Amplification by Stimulated Emission of Radiation.
• Components:
  • Light Source: Starts the process (often a diode in laser pointers).
  • Amplifying Medium: A gas or solid material that increases light via stimulated emission.
  • Mirrors: Placed at both ends to bounce light back and forth.
• Functions of Mirrors:
  • Increases time in the amplifying medium to generate more light.
  • Facilitates constructive interference so waves are in the same phase and have the same wavelength.
• Coherent Light: The output of a laser where all light waves have identical frequency and phase, resulting in high intensity.
• Industrial and Medical Uses: Laser welding, laser surgery, and laser printing.