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1 Sunlight 1 Sunlight 2 Observations about Sunlight Sunlight Sunlight appears whiter than most light Sunlight makes the sky appear blue Sunlight becomes redder at sunrise and sunset It reflects from many surfaces, even nonmetals It bends and separates into colors in materials Turn off all electronic devices Sunlight 3 5 Questions about Sunlight Sunlight 4 Question 1 1. Why does sunlight appear white? 2. Why does the sky appear blue? 3. How does a rainbow break sunlight into colors? 4. Why are soap bubbles and oil films so colorful? 5. Why do polarizing sunglasses reduce glare? Q: Why does sunlight appear white? A: We perceive 5800 K thermal light as white Light is a class of electromagnetic waves Single-frequency light has a rainbow color A thermal mixture of rainbow colors can look white Sunlight 5 Spectrum of Sunlight Sunlight 6 Question 2 Sunlight is thermal radiation heat from the sun Charges in the sun s hot photosphere jitter thermally Accelerating charges emits electromagnetic waves The sun emits a black-body body spectrum at 5800 K We perceive thermal light at 5800 K as white Q: Why does the sky appear blue? A: Air particles Rayleigh-scatter bluish light best Rayleigh scattering occurs when passing sunlight polarizes tiny particles in the air, this alternating polarization reemits emits light waves, so air particles scatter light they they absorb and reemit it. 1

2 Sunlight 7 Rayleigh Scattering Sunlight 8 Question 3 Air particles are so small that they are much less ¼ the wavelength of light poor antennas for light scatter long-wavelengths l n (reds) particularly rl poorly scatter short-wavelengths (violets) somewhat better Rayleigh scattered sunlight appears bluish Unscattered sunlight (solar disk) appears reddish Effect is strongest at sunrise and sunset Q: How does a rainbow break sunlight into colors? A: Rainbow colors take different paths in raindrops Sunlight slows while it passes through matter Light waves electrically polarize the matter That polarization delays and slows the light wave Index of refraction = reduction factor for light s speed Index of refraction varies slightly with color Sunlight 9 Light at Interfaces Sunlight 10 Light and Dispersion When light changes speed at an interface, relationship between electric & magnetic fields changes it refracts its path bends as it cross the interface it bends toward the perpendicular if it slows down it bends away from the perpendicular if it speeds up it reflects part of it bounces off the interface the reflection is almost perfect for metal surfaces the reflection is partial for insulator surfaces The rainbow colors of light in sunlight have different frequencies polarize material slightly differently and therefore r travel at slightly different speeds Index of refraction depends slightly on color Violet light usually travels slower than red light violet light usually refracts more than red violet light usually reflects more than red Sunlight 11 Rainbows Sunlight 12 Question 4 Occur when sunlight encounters water droplets and undergoes refraction, reflection, and dispersion. Q: Why are soap bubbles and oil films so colorful? A: They display color-dependent interference effects Light waves following different paths can interfere The two partial reflections from a soap or oil film can interfere Different colors of light can interfere differently 2

3 Sunlight 13 Question 5 Sunlight 14 Reflection of Polarized Light Q: Why do polarizing sunglasses reduce glare? A: Glare is mostly horizontally polarized light Sunlight is a uniform mix of polarizations When sunlight partially reflects at a shallow angle its different polarizations reflect differently it becomes polarized it it is no longer a uniform mix Polarizing sunglasses block horizontal polarization Polarization affects angled reflections When light s electric field is parallel to a surface there is a large fluctuating surface polarization and thus a strong reflection. When electric field is perpendicular to a surface there is a small fluctuating surface polarization and thus a weak reflection. Glare is mostly polarized parallel to the surface Sunlight 15 Polarization and Sunlight Sunlight 16 Summary about Sunlight Polarizing sunglasses block horizontally polarized light and thus block glare from horizontal surfaces. Rayleigh scattering has polarizing i effects, so much of the blue sky is polarized light, too. Polarizing sunglasses darken much of the sky Sunlight is thermal light at about 5800 K It undergoes Rayleigh scattering in the air It bends and reflects from raindrops It interferes colorfully in soap and oil films It reflects in a polarizing fashion from surfaces Sunlight 17 Sunlight 18 Observations about They often take moment to turn on They come in a variety of colors, including white They are often whiter than incandescent bulbs They last longer than incandescent bulbs They sometimes hum loudly They flicker before they fail completely Turn off all electronic devices 3

4 Sunlight 19 4 Questions about 1. Why phase out incandescent lightbulbs? 2. How can colored lights mix so we see white? 3. Why does a neon lamp produce red light? 4. How can white light be produced without heat? 5. How do gas discharge lamps produce light? Sunlight 20 Question 1 Q: Why phase out incandescent lightbulbs? A: Because they waste too much electric power. Incandescent lightbulb is a thermal light source with a relatively low filament temperature of 2700 K It emits mostly invisible infrared light Less than 10% of its thermal power is visible light Non-thermal light sources can be more efficient Sunlight 21 Question 2 Sunlight 22 Question 3 Q: How can colored lights mix so we see white? A: Primary colors of light trick our vision. We have three groups of light-sensing g cone cells Their peak responses are to red, green, and blue light Those are therefore the primary colors of light Mixtures of primary colors can make us see any color Q: Why does a neon lamp emit red light? A: Neon s quantum structure dictates light emission Electrons obey the rules of quantum physics In matter, electrons exist as quantum standing waves three-dimensional patterns of nodes and antinodes each wave cycles in place it does not change with time In atoms, those standing waves are called orbitals In solids, those standing waves are called levels Quantum structure dictates atom s light emission Sunlight 23 Quantum Physics Sunlight 24 Electrons in Matter Classical physics (pre-1900) thought that everything in nature is a particle or a wave electrons, atoms, and billiard balls are particles light and sound are waves Modern physics (post-1900) recognizes that everything in nature is both particle and wave things are most wave-like when they are left alone things are most particle-like like when they interact Electrons exist in matter as quantum standing waves have energies that are set by their specific waves obey the Pauli exclusion principle: No two indistinguishable Fermi particles ever occupy the same quantum wave have two distinguishable states: spin-up or spin-down can occupy each wave in 1 s or 2 s, but no more than 2 tend to occupy lowest energy waves, 2 per wave 4

5 Sunlight 25 Electrons in a Neon Atom Sunlight 26 Neon Electrons in any atom tend to settle into the lowest energy orbitals cannot be more than 2 to an orbital Lowest energy arrangement is atom s ground state Higher energy arrangements are atom s excited states In a neon atom, the nucleus has 10 protons electrical neutrality requires it to have 10 electrons ground state has electrons in 5 lowest-energy energy orbitals Neon lamp metal electrodes inject free charges into dilute neon plasma forms a vapor of charged particles electric field causes current to flow in the plasma current is mostly electrons streaming toward positive electrons often collide violently with neon atoms Sunlight 27 Neon Lamps and Excited States Sunlight 28 Light from Atoms Collisions in the plasma occasionally ionize neon atoms, sustaining the plasma cause electronic excitations of the neon atoms In a neon atom, electrons normally occupy ground state orbitals collisions can shift electrons to higher energy orbitals light emission can return them to lower energy orbitals Atoms interact with light via radiative transitions Radiative transition that emits light is fluorescence The quantum physics of light: Light travels as a wave (diffuse rippling fields) Light is emitted or absorbed as a particle (a photon). A photon carries a specific amount of energy Photon energy = Planck constant frequency An atom s orbitals differ by specific energies Orbital energy differences set the photon energies Excited atom emits a specific spectrum of photons Sunlight 29 Atomic Fluorescence Sunlight 30 Question 4 Photon energy is the difference in orbital energies Small energy differences infrared (IR) photons Moderate energy differences red photons Big energy differences blue photons Even bigger energy differences ultraviolet (UV) photons Each atom has its own fluorescence spectrum Neon s fluorescence spectrum is dominated by red light Q: How can white light be produced without heat? A: Synthesize the proper mixture of primary colors. Fluorescent tubes use a discharge in mercury gas to produce UV light UV light causes phosphors on the tube wall to glow phosphors synthesize white light from primary colors 5

6 Sunlight 31 Phosphors Sunlight 32 Starting Fluorescent Lamps A mercury discharge emits mostly UV light A phosphor can convert UV light to visible Absorb a UV photon, emit visible photon. Missing energy usually becomes thermal energy. Fluorescent lamps use white phosphors They imitate thermal whites at 2700 K, 5800 K, etc. Specialty lamps use colored phosphors Blue, green, yellow, orange, red, violet, etc. Starting a discharge requires electrons in the gas Those electrons can be injected into the gas by heated filaments with special coatings or by high voltages Once discharge starts, it can sustain the plasma Starting the discharge damages the electrodes Atoms are sputtered off the electrodes Damage limits the number of times a lamp can start Sunlight 33 Stabilizing Fluorescent Lamps Sunlight 34 Question 5 Gas discharges are electrically unstable Gas is initially insulating Once discharge is started, gas become a conductor The more current it carries, the better it conducts Current tends to skyrocket out of control Stabilizing discharge requires ballast Inductor ballast (old, 60 Hz, tend to hum) Electronic ballast (new, high-frequency, silent) Q: How do gas discharge lamps produce light? A: The discharge emits atomic fluorescence light Sunlight 35 Low-Pressure Sunlight 36 High Pressure Effects Mercury gas has its resonance line in the UV Low-pressure mercury lamps emit mostly UV light Some gases have resonance lines in the visible Low-pressure sodium vapor discharge lamps emit sodium s yellow-orange orange resonance light, so they are highly energy efficient but extremely monochromatic and hard on the eyes. High pressures broaden each spectral line Collisions occur during photon emissions, so frequency and wavelength become smeared out. Collision i energy shifts the photon energy Radiation trapping occurs at high atom densities Atoms emit resonance radiation very efficiently Atoms also absorb resonance radiation efficiently Resonance radiation photons are trapped in the gas Energy must escape discharge via other transitions 6

7 Sunlight 37 High-Pressure At higher pressures, new spectral lines appear High-pressure sodium vapor discharge lamps emit a richer spectrum of yellow-orange orange colors, are still quite energy efficient, but are less monochromatic and easier on the eyes. High-pressure mercury discharge lamps emit a rich, bluish-white spectrum, with good energy efficiency. Adding metal-halides adds red to improve whiteness. Sunlight 38 Summary about Thermal light sources are energy inefficient Discharge lamps produce more light, less heat They carefully assemble their visible spectra They use atomic fluorescence to create light Some include phosphors to alter colors 7