Astro 201 Sept 14 2010 Read Hester Chapter 4 Chaos and Fractal information on class web page On Line quiz 3 available after class due next Tuesday before class HW 3 on line Today Light LIGHT and Telescopes LIGHT Astronomers study the Universe by analyzing the light from cosmic objects LIGHT Maxwell s equations predict that moving charged particles will produce periodic waves of electric and magnetic field Periodic function Travels through space Ocean wave fluctuation in height of water Sound wave fluctuation in air pressure Electromagnetic wave fluctuation in electric and magnetic fields Wavelength Frequency c The speed of a wave equals wavelength times frequency c f c for celeritas the Latin word for speed The speed of light in a vacuum is ALWAYS c 300 000 km sec 186 000 miles sec Figure 4 6 Wavelength Speed Frequency and Amplitude Light is made of PARTICLES Light shows some properties of particles such as the photoelectric effect Particles of light called PHOTONS kick electrons out of atoms The ENERGY of a photon is related to the FREQUENCY of the wave E h f E energy of photon f frequency of light wave h Planck s constant a number Light forms a spectrum from short to long wavelength Roy G Biv Optical or Visible Light ROY G BIV Bright at all wavelengths Bright at specific wavelengths Bright at all except some specific wavelengths An example of a Continuum Spectrum Blackbody A blackbody is an object that absorbs all light that falls on it It radiates light which has a distinctive spectrum specified by its temperature The peak of the spectrum for blackbodies with T 98 6 F is in the infrared part of the spectrum T 6000 degree black bodies have peaks in the visible part of the spectrum e g the SUN Incandescent light bulb filament 2000 3000 deg Stars have spectra that are more or less like those of blackbodies Absorption and Emission Lines in Spectra Produced when light interacts with atoms A nucleus consisting of protons and usually neutrons is surrounded by a cloud of electrons HYDROGEN One Proton one electron proton electron Behavior on subatomic scales is governed by QUANTUM MECHANICS Rule electrons can only exist in orbits of particular energy Small orbit low energy big orbit high energy Electron falls from high energy to low energy orbit energy is carried away by a photon 656 3 nm 121 6 nm 102 6 nm Photon has a fixed ENERGY corresponding to a specific WAVELENGTH Consider a hot low density glob of hydrogen gas 656 3 nm 3 2 486 1 nm 4 2 434 0 nm 5 2 Light is emitted with wavelengths or frequencies corresponding to energy jumps between electron orbits 1 Hot low density gas produces an emission line spectrum 5 2 4 2 3 2 Spectrum of hydrogen at visible wavelengths Carina Nebula a cloud of hot low density gas about 7000 light years away Its reddish color comes from the 656 3 nm emission line of hydrogen A cool low density glob of hydrogen gas in front of a light source Light is absorbed at specific wavelengths corresponding to energy jumps between electron orbits 2 Cool low density gas produces an absorption line spectrum 5 2 4 2 3 2 Spectrum of hydrogen at visible wavelengths Every type of atom has a unique spectrum The spectrum of the Carina Nebula Hydrogen line at 656 3 nm The RADIAL VELOCITY of an object is found from the DOPPLER SHIFT of its spectrum Radial velocity how fast an object is moving toward you or away from you DOPPLER SHIFT If the wave source moves toward you or away from you the wavelength changes Christian Doppler 1803 1853 A modern recreation of Doppler s famous experiment Doppler effect of sound http www youtube com watch v ZlCcX697Twg The reason for Doppler shifts Wave crests are bunched up ahead of wave source stretched out behind wave source Figure 4 16 The Doppler shift of light If a light source is moving TOWARDS you the wavelength appears shorter called blueshift If the light source is moving AWAY from you wavelength is longer called redshift Doppler shifts are easily detected in emission or absorption line spectra Figure 4 17 Doppler shifts of astronomical objects Size of Doppler shift is proportional to radial velocity V 0 c observed wavelength shift 0 0 wavelength if source isn t moving V radial velocity of moving source c speed of light 300 000 km sec One of the nifty applications of the Doppler effect has been the detection of Planets orbiting other stars Extrasolar Planets The star and planet orbit around their center of mass periodic red and blue shifts of the lines in the star s spectrum To date about 358 planets have been discovered mostly larger than Jupiter Another nifty application of the Doppler effect Doppler radar to predict weather Not so nifty Police radar guns to catch you speeding