Interactions of Light with Matter: Understanding the Properties of Light and Matter - Prof, Study notes of Astronomy

Download Interactions of Light with Matter: Understanding the Properties of Light and Matter - Prof and more Study notes Astronomy in PDF only on Docsity! 1 Chapter 5 Light and Matter: Reading Messages from the Cosmos 5.1 Light in Everyday Life Our goals for learning: • How do we experience light? • How do light and matter interact? How do we experience light? • The warmth of sunlight tells us that light is a form of energy • We can measure the flow of energy in light in units of watts: 1 watt = 1 joule/s 2 Colors of Light • White light is made up of many different colors How do light and matter interact? • Emission • Absorption • Transmission – Transparent objects transmit light – Opaque objects block (absorb) light • Reflection or Scattering Reflection and Scattering Mirror reflects light in a particular direction Movie screen scatters light in all directions 5 Properties of Waves • Wavelength is the distance between two wave peaks • Frequency is the number of times per second that a wave vibrates up and down wave speed = wavelength x frequency Light: Electromagnetic Waves • A light wave is a vibration of electric and magnetic fields • Light interacts with charged particles through these electric and magnetic fields Wavelength and Frequency wavelength x frequency = speed of light = constant 6 Particles of Light • Particles of light are called photons • Each photon has a wavelength and a frequency • The energy of a photon depends on its frequency Wavelength, Frequency, and Energy λ x f = c λ = wavelength , f = frequency c = 3.00 x 108 m/s = speed of light E = h x f = photon energy where h = 6.626 x 10-34 joule x s Special Topic: Polarized Sunglasses • Polarization describes the direction in which a light wave is vibrating • Reflection can change the polarization of light • Polarized sunglasses block light that reflects off of horizontal surfaces 7 What is the electromagnetic spectrum? Thought Question The higher the photon energy… a) the longer its wavelength. b) the shorter its wavelength. c) energy is independent of wavelength. What have we learned? • What is light? – Light can behave like either a wave or a particle – A light wave is a vibration of electric and magnetic fields – Light waves have a wavelength and a frequency – Photons are particles of light. • What is the electromagnetic spectrum? – Human eyes cannot see most forms of light. – The entire range of wavelengths of light is known as the electromagnetic spectrum. 10 Phases and Pressure • Phase of a substance depends on both temperature and pressure • Often more than one phase is present How is energy stored in atoms? • Electrons in atoms are restricted to particular energy levels Ground State Excited States Energy Level Transitions • The only allowed changes in energy are those corresponding to a transition between energy levels AllowedNot Allowed 11 What have we learned? • What is the structure of matter? – Matter is made of atoms, which consist of a nucleus of protons and neutrons surrounded by a cloud of electrons • What are the phases of matter? – Adding heat to a substance changes its phase by breaking chemical bonds. – As temperature rises, a substance transforms from a solid to a liquid to a gas, then the molecules can dissociate into atoms – Stripping of electrons from atoms (ionization) turns the substance into a plasma What have we learned? • How is energy stored in atoms? – The energies of electrons in atoms correspond to particular energy levels. – Atoms gain and lose energy only in amount corresponding to particular changes in energy levels. 5.4 Learning from Light Our goals for learning: • What are the three basic types of spectra? • How does light tell us what things are made of? • How does light tell us the temperatures of planets and stars? • How do we interpret an actual spectrum? 12 What are the three basic types of spectra? Continuous Spectrum Emission Line Spectrum Absorption Line Spectrum Spectra of astrophysical objects are usually combinations of these three basic types Three Types of Spectra Continuous Spectrum • The spectrum of a common (incandescent) light bulb spans all visible wavelengths, without interruption 15 Chemical Fingerprints • Because those atoms can absorb photons with those same energies, upward transitions produce a pattern of absorption lines at the same wavelengths Chemical Fingerprints • Each type of atom has a unique spectral fingerprint 16 Chemical Fingerprints • Observing the fingerprints in a spectrum tells us which kinds of atoms are present Example: Solar Spectrum 17 Energy Levels of Molecules • Molecules have additional energy levels because they can vibrate and rotate Energy Levels of Molecules • The large numbers of vibrational and rotational energy levels can make the spectra of molecules very complicated • Many of these molecular transitions are in the infrared part of the spectrum Thought Question Which letter(s) labels absorption lines? A B C D E 20 Thought Question Why don’t we glow in the dark? a) People do not emit any kind of light. b) People only emit light that is invisible to our eyes. c) People are too small to emit enough light for us to see. d) People do not contain enough radioactive material. How do we interpret an actual spectrum? • By carefully studying the features in a spectrum, we can learn a great deal about the object that created it. What is this object? Reflected Sunlight: Continuous spectrum of visible light is like the Sun’s except that some of the blue light has been absorbed - object must look red 21 What is this object? Thermal Radiation: Infrared spectrum peaks at a wavelength corresponding to a temperature of 225 K What is this object? Carbon Dioxide: Absorption lines are the fingerprint of CO2 in the atmosphere What is this object? Ultraviolet Emission Lines: Indicate a hot upper atmosphere 22 What is this object? Mars! What have we learned? • What are the three basic type of spectra? – Continuous spectrum, emission line spectrum, absorption line spectrum • How does light tell us what things are made of? – Each atom has a unique fingerprint. – We can determine which atoms something is made of by looking for their fingerprints in the spectrum. What have we learned? • How does light tell us the temperatures of planets and stars? – Nearly all large or dense objects emit a continuous spectrum that depends on temperature. – The spectrum of that thermal radiation tells us the object’s temperature. • How do we interpret an actual spectrum? – By carefully studying the features in a spectrum, we can learn a great deal about the object that created it. Measuring Redshift Measuring Redshift eternity Measuring Velocity 25 26 Measuring Velocity How does light tell us the rotation rate of an object? • Different Doppler shifts from different sides of a rotating object spread out its spectral lines Spectrum of a Rotating Object • Spectral lines are wider when an object rotates faster 27 What have we learned? • How does light tell us the speed of a distant object? – The Doppler effect tells us how fast an object is moving toward or away from us. • Blueshift:objects moving toward us • Redshift: objects moving away from us • How does light tell us the rotation rate of an object? – The width of an object’s spectral lines can tell us how fast it is rotating