Chapter 4Properties of LightProperties of LightWavelengths and FrequencyWhich of the following best describes light?Slide 6White LightThe Electromagnetic SpectrumInfrared and UltravioletThe Rest of the EM SpectrumEnergy Carried by EM RadiationSlide 12Slide 13Slide 14Slide 15Slide 16The Kelvin ScaleTemperature/RadiationThe Chemical ElementsElectron OrbitalsSlide 21Light and AtomsLight and AtomsSlide 24A photon with wavelength 500 nm strikes an atom. What happens?Light and AtomsLight and AtomsTypes of SpectraAstronomical SpectraSlide 30Absorption in the AtmosphereThe Doppler ShiftChapter 4Light and AtomsProperties of Light•Light is radiant energy; it needs no medium to travel through.•Light has a speed limit (3*108 m/s, about 186,000 miles per second)•Wave model of lightLight is a wave made up of interlinked waves of magnetic and electric energy.•Photon model of lightLight is a stream of particles/packets of energy called photons•Wave-Particle DualityLight is both a particle and a wave.Scientists use whichever model best describes a particular phenomenon.Properties of Light•Brightness: Total amount of energy carried by the light.•Wave model: Related to the height of the wave.•Photon model: Related to the number of photons.•Color: Relates to the wavelength of the light (Mostly wave model)•Red light wavelength = 4*10-7 m (400 nanometers)•Blue light wavelength = 7*10-7 m (700 nanometers)•The range of colors (wavelengths) humans can see is called the “visible spectrum”Wavelengths and Frequency•Wavelength: The physical distance between successive crests of the wave.•Frequency: The number of oscillations of the wave in a given amount of time (usually 1 second, called a “Hertz”)•Wavelength * Frequency = Speed of the wave (speed of light)Larger wavelengths ————><———— Larger FrequenciesV I B G Y O RxWhich of the following best describes light?A. Light is a waveB. Light is a particleC. Light is both a wave and a particleD. Light is neither a wave nor a particleTwo light sources have the same brightness but are different colors. Which of the following are true?A. The wavelengths of light are the sameB. The number of photons produced by each source are the sameC. Both A and BD. Neither A nor BWhite Light•White light is a mixture of all colors of the visible spectrum.The Electromagnetic Spectrum•Visible Light is only one kind of electromagnetic radiation.•The other kinds are exactly like visible light (wave-particle duality, can be called photons, travel at the speed of light, etc), except that they have different wavelengths (and therefore frequencies).Infrared and UltravioletInfrared•Discovered by Herschel while trying to measure which color of (visible) light carried more heat. The hottest color was just past red on the EM spectrum.•Longer wavelengths than visible light. First discovery outside visible spectrum.•Our skin can “feel” infrared radiation as heat.Ultraviolet•Discovered in a similar way, silver chloride blackened most just outside purple.•UV has shorter wavelengths than visible light.•Causes sunburns.The Rest of the EM SpectrumRadio Waves and MicrowavesX Rays and Gamma RaysVery small wavelengths.Very large frequencies.Very large wavelengths.Very small frequencies.Gate X Usually Lets In Most RadiationEnergy Carried by EM RadiationE = h*c λThe energy of light depends on its wavelength.Smaller wavelengths carry more energy.Understand the concept, and be able to dobasic calculations. I will give you values for c and h.Order the following colors in INCREASING wavelength.Red, Yellow, GreenA. Red, Yellow, GreenB. Red, Green, YellowC. Yellow, Green, RedD. Green, Red, YellowE. Green, Yellow, RedOrder the following colors in INCREASING Frequency.Red, Yellow, GreenA. Red, Yellow, GreenB. Red, Green, YellowC. Yellow, Green, RedD. Green, Red, YellowE. Green, Yellow, RedOrder the following colors in INCREASING Energy.Red, Yellow, GreenA. Red, Yellow, GreenB. Red, Green, YellowC. Yellow, Green, RedD. Green, Red, YellowE. Green, Yellow, RedOrder X-rays, microwaves, and green light in terms of increasing wavelength.A. X-rays, microwaves, green lightB. Microwaves, green light, X-raysC. X-rays, green light, microwavesD. Green light, microwaves, X-raysOrder X-rays, microwaves, and green light in terms of increasing frequency/energy.A. X-rays, microwaves, green lightB. Microwaves, green light, X-raysC. X-rays, green light, microwavesD. Green light, microwaves, X-raysThe Kelvin Scale•“Absolute” temperature scale used by scientists.•Zero means zero.•It’s just “Kelvin,” not “degrees Kelvin”Temperature/Radiation•The higher the temperature of an object, the more strongly that object radiates at shorter wavelengths (high frequencies, high energies).•Only applies to radiating bodies (blackbodies), not everyday objects.The Chemical Elements•Dense core of protons (positive) and neutrons (no charge) held together by the “strong force.”•Electrons (negative) in “orbit.”•The number of protons determines which element it is.•Different numbers of neutrons makes an “isotope.” Some isotopes are stable, some decay radioactively.•Different number of electrons and protons makes an “ion.”Electron Orbitals•Very very tiny, around 10-10 meters.•Because it’s so small, the standard rules we know from larger objects do not apply.•Electron orbits are only permitted at certain “sizes.” We say that these sizes are “quantized.”•Electron orbits are “smeared”•Electrons can change energy levels.•Neils Bohr: “Anyone who is not shocked by quantum theory has not understood it.”•An analogy will be helpful.Painters can only be oncertain levels of the scaffolding,not anywhere in between.Light and Atoms•If all the electrons are in the lowest state they can be in, the atom is in the “ground state.” Anything else, and the atom is “excited.”•Energy is conserved, so when the electrons change orbitals, that energy must go/come from somewhere.•That “somewhere” is light (electromagnetic radiation)Emission: An electron drops to alower orbital, releasing energy asa photon (electromagnetic energy)Light and Atoms•Absorption: An atom absorbs a photon (electromagnetic energy) and an electron jumps up to a higher orbit.•Absorption can only occur if the energy of the photon matches the energy difference between the orbitals.In what ways are electron