The Nature of Light: Electromagnetic Waves and Blackbody Radiation - Prof. Jie Zhang, Study notes of Astronomy

Download The Nature of Light: Electromagnetic Waves and Blackbody Radiation - Prof. Jie Zhang and more Study notes Astronomy in PDF only on Docsity! The Nature of Light Chapter Five — Pd rl a ASTR 111 – 003 Fall 2007 Lecture 05 Oct. 01, 2007 Introducing Astronomy (chap. 1-6) Introduction To Modern Astronomy I: Solar System Ch1: Astronomy and the Universe Ch2: Knowing the Heavens Ch3: Eclipses and the Motion of the Moon Ch4: Gravitation and the Waltz of the Planets Ch5: The Nature of Light Ch6: Optics and Telescope Planets and Moons (chap. 7-15) Chap. 16: Our Sun Chap. 28: Search for Extraterrestrial life 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 openings Electromagnetic 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 in space in the form of waves at a speed of 3.0 X 105 km/s Electromagnetic Waves • 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 1014 Hz Electromagnetic Waves λ ν c= ν: Frequency (in Hz) λ: Wavelength (in meter) c: Speed of light = 3 x 108 m/s Heated iron bar: as the temperature increases – The bar glows more brightly – The color of the bar also changes Blackbody 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 intensity Blackbody curve Blackbody Radiation (Box 5-1) Temperature Scales Temperature in unit of Kelvin is often used in physics TK = TC +273 TF = 1.8 (TC+32) Zero Kelvin is the absolute minimum of all temperatures Wien’s Law •Wien’s law states that the wavelength of maximum emission of a blackbody is inversely proportional to the Kelvin temperature of the object For example – The Sun, λmax = 500 nm T = 5800 K – Human body at 100 F, what is λmax? (Box 5-2) Wien’s Law Sirius, 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? 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 10–34 J 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 10–19 J – or E = 1.96 eV – eV: electron volt, a small energy unit = 1.602 x 10–19 J Dual properties of Light (Box 5-3) Planck’s Law The 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? Each chemical element has its own unique set of spectral lines. Spectral Lines Kirchhoff’s Laws on Spectra • Three different spectra: continuous spectrum, emission-line spectrum, and absorption line spectrum Kirchhoff’s Laws on Spectra • Law 1- Continuous spectrum: a hot opaque body, such as a perfect blackbody, produce a continuous spectrum – a complete rainbow of colors without any spectral line • Law 2 – emission line spectrum: a hot, transparent gas produces an emission line spectrum – a series of bright spectral lines against a dark background • Law 3 – absorption line spectrum: a relatively cool, transparent gas in front of a source of a continuous spectrum produces an absorption line spectrum – a series of dark spectral lines amongst the colors of the continuous spectrum. Further, the dark lines of a particular gas occur at exactly the same wavelength as the bright lines of that same gas. • Electrons occupy only certain orbits or energy levels • When an electron jumps from one orbit to another, it emits or absorbs a photon of appropriate energy. • The energy of the photon equals the difference in energy between the two orbits. Bohr’s Model of Atom Bohr’s Model of Hydrogen Atom • Absorption is produced when electron absorbs incoming photon and jumps from a lower orbit to a higher orbit • Emission is produced when electron jumps from a higher orbit to a lower orbit and emits a photon of the same energy Bohr’s Model of Atom 0502_Absorption_Photon.swf FLASH • The strongest hydrogen spectral line from the Sun, Hα line at 656 nm, is caused by electron-transition between n=3 orbit and n=2orbit • Balmer series lines: between n-2 orbit and higher orbits (n=3, 4, 5,…) • Lyman series lines: between n=1 orbit and higher orbits (n=2, n=3, n=4,…) (UV) • Paschen series lines: between n=3 orbit and higher orbits (n=4, n=5, n=6,…) (IR) Bohr’s Model of Atom (Box 5-6) Doppler Effect In the spectrum of the star Vega, the prominent Hα spectra line of hydorgen has a wavelength λ = 656.255 nm. At laboratory, this line has a wavelength λ0 = 656.285 nm. What can we conclude about the motion of Vega? Final Notes on Chap. 5 • There are 9 sections. All section are covered Advanced Question Chap. 5, Q30 in P125 Jupitor’s moon Io has an active volcano Pele whose temperature can be as high as 320°C. (a) What is the wavelength of maximum emission for the volcano at this temperature? In what part of the electromagnetic spectrum is this? (b) The average temperature of Io’s surface is - 150 °C. Compared with a square meter of surface at this temperature, how much more energy is emitted per second from each square meter of Pele’s surface?