The Nature of LightIntroduction To Modern Astronomy I: Solar SystemSpeed of LightSpeed of LightElectromagnetic WavesElectromagnetic WavesElectromagnetic WavesElectromagnetic WavesElectromagnetic WavesBlackbody RadiationBlackbody Radiation(Box 5-1) Temperature ScalesWien’s Law(Box 5-2) Wien’s LawStefan-Boltzmann Law(Box 5-2) Stefan-Boltzmann LawDual properties of Light: (1) wave and (2) particle(Box 5-3) Planck’s LawSpectral LinesKirchhoff’s Laws on SpectraKirchhoff’s Laws on SpectraStructure of Atom(Box 5-6) Doppler Effect Final Notes on Chap. 5Advanced Question Chap. 5, Q30 in P125The Nature of LightChapter FiveASTR 111 – 003 Fall 2007Lecture 05 Oct. 01, 2007Introducing Astronomy (chap. 1-6)Introduction To Modern Astronomy I:Solar SystemCh1: Astronomy and the UniverseCh2: Knowing the HeavensCh3: Eclipses and the Motion of the MoonCh4: Gravitation andthe Waltz of the PlanetsCh5: The Nature of LightCh6: Optics and TelescopePlanets and Moons (chap. 7-15)Chap. 16: Our SunChap. 28: Search forExtraterrestrial life• The speed of light in the vacuum– C = 299,792.458 km/s, or – C = 3.00 X 105km/s = 3.00 X 108m/s• It takes the light 500 seconds traveling 1 AU.• It takes the light 4.2 years to the nearest star Proxima Centauri• Milky way diameter ~ 100,000 lysSpeed of LightSpeed of Light• In 1676, Danish astronomer Olaus Rømerdiscovered that the exact time of eclipses of Jupiter’s moons depended on the distance of Jupiter to Earth • The variation is about 16.6 minutes (or 1000 seconds) • This happens because it takes varying times for light to travel the varying distance between Earth and Jupiter (varying by up to 2 AU)Electromagnetic Waves• Newton (in 1670) found that the white light from the Sun is composed of light of different color, or spectrum• Young’s Double-Slit Experiment (in 1801) indicated light behaved as a wave• The alternating black and bright bands appearing on the screen is analogous to the water waves that pass through a barrier with two openingsElectromagnetic Waves• The nature of light is electromagnetic radiation• In the 1860s, James Clerk Maxwell succeeded in describing all the basic properties of electricity and magnetism in four equations: the Maxwell equations of electromagnetism.• Maxwell showed that electric and magnetic field should travel inspace in the form of waves at a speed of 3.0 X 105km/sElectromagnetic Waves• Visible light falls in the 400 to 700 nm range• In the order of decreasing wavelength – Radio waves: > 10 cm– Microwave: 1 mm – 10 cm– Infrared: 700 nm – 1mm– Visible light: 400 nm – 700 nm– Ultraviolet: 10 nm – 400 nm– X-rays: 0.01 nm - 10 nm– Gamma rays: < 0.01 nmElectromagnetic WavesElectromagnetic WavesUses of Non-visible Electromagnetic Radiation•Example– FM radio, e.g., 103.5 MHz (WTOP station) => λ = 2.90 m– Visible light, e.g., red 700 nm => ν = 4.29 X 1014HzElectromagnetic Wavesλνc=ν: Frequency (in Hz)λ: Wavelength (in meter)c: Speed of light = 3 x 108m/sHeated iron bar: as the temperature increases– The bar glows more brightly– The color of the bar also changesBlackbody Radiation• Blackbody curve: the intensities of radiation emitted at various wavelengths by a blackbody at a given temperature– The higher the temperature, the shorter the peak wavelength– The higher the temperature, the higher the intensityBlackbody curveBlackbody Radiation•A blackbody is a hypothetical object that is a perfect absorber of electromagnetic radiation at all wavelengths– The radiation of a blackbody is entirely the result of its temperature– A blackbody does not reflect any light at all• Most dense objects can be regarded as a blackbody– e.g., a star, a planet, a human body– but not a thin cloud, a layer of thin gas (lights get through)Blackbody Radiation• The Sun’s radiation is remarkably close to that from a blackbody at a temperature of 5800 KBlackbody RadiationThe Sun as a BlackbodyA Human Body as a Blackbody(Box 5-1) Temperature ScalesTemperature in unit of Kelvin is often used in physicsTK= TC+273TF= 1.8 (TC+32)Zero Kelvin is the absolute minimum of all temperaturesWien’s Law•Wien’s law states that the wavelength of maximum emission of a blackbody is inversely proportional to the Kelvin temperature of the objectFor example– The Sun, λmax= 500 nm Æ T = 5800 K– Human body at 100 F, what is λmax?(Box 5-2) Wien’s LawSirius, the brightest star (also called dog star, in Canis Major) in the night sky, has a surface temperature of 10,000 K. Find the wavelength at which Sirius emits most intensely?Stefan-Boltzmann Law• The Stefan-Boltzmann law states that a blackbody radiates electromagnetic waves with a total energy flux F directly proportional to the fourth power of the Kelvin temperature T of the object:F = σT4• F = energy flux, in joules per square meter of surface per second• σ = Stefan-Boltzmann constant = 5.67 X 10-8W m-2K-4• T = object’s temperature, in kelvins• 1 J = kinetic (energy) of a 2 kg mass at a speed of 1 m/s• 1 W = 1 J/s (power)• F: energy flux: J/m2/s (flux)(Box 5-2) Stefan-Boltzmann LawSirius, the brightest star (also called dog star, in Canis Major) in the night sky, has a surface temperature of 10,000 K. How does the energy flux from Sirius compare to the Sun’s energy flux?Dual properties of Light: (1) wave and (2) particle• Light is an electromagnetic radiation wave, e.g, Young’s double slit experiment• Light is also a particle-like packet of energy– A light packet is called photon– The energy of photon is related to the wavelength of light• Light has a dual personality; it behaves as a stream of particles like photons, but each photon has wavelike properties• Planck’s law relates the energy of a photon to its wavelength (frequency)– E = energy of a photon– h = Planck’s constant = 6.625 x 10–34J s– c = speed of light– λ= wavelength of light• Energy of photon is inversely proportional to the wavelength of light• Example: 633-nm red-light photon– E = 3.14 x 10–19J– or E = 1.96 eV– eV: electron volt, a small energy unit = 1.602 x 10–19JDual properties of Light(Box 5-3) Planck’s LawThe bar-code scanners used at supermarket emit orange-red light of wavelength 633 nm and consume a power 1 mW. Calculate how many photons are emitted by one such scanner per second?Spectral Lines• The Sun’s spectrum: in addition to the rainbow-colored