Presentation on theme: "Electromagnetic Radiation"— Presentation transcript:
1 Electromagnetic Radiation (How we get information about the cosmos)Examples of electromagnetic radiation?LightInfraredUltravioletMicrowavesAM radioFM radioTV signalsCell phone signalsX-rays
2 The Electromagnetic Spectrum 1 nm = m , 1 Angstrom = mc =l n
3 Refraction of lightAll waves bend when they pass through materials of different densities. When you bend light, bending angle depends on wavelength, or color.
4 The "Inverse-Square" Law Applies to Radiation Each square gets 1/4 of the lightEach square gets 1/9 of the light1D2apparent brightness αα means “is proportional to”. D is the distance between source and observer.
5 Why is the Sun yellow? The Black-Body Spectrum We form a "spectrum" by spreading out radiation according to its wavelength (e.g. using a prism for light).What does the spectrum of an astronomical object's radiation look like?Many objects (e.g. stars) have roughly a "Black-body" spectrum:Asymmetric shapeBroad range of wavelengthsor frequenciesHas a peakBrightnessFrequencyalso known as the Planck spectrum.
6 Approximate black-body spectra of stars of different temperature cold dustaverage star (Sun)Brightnessinfrared visible UV“cool" starsvery hot starsinfrared visible UVfrequency increases, wavelength decreases
7 Laws Associated with the Black-body Spectrum Wien's Law:1Tλmax energy α(wavelength at which most energy is radiated is longer for cooler objects)Stefan's Law:Energy radiated per cm2 of area on surface every second α T 4(T = temperature at surface)1 cm2
8 The total energy radiated from entire surface every second is called the luminosity. ThusLuminosity = (energy radiated per cm2 per sec) x (area of surface in cm2)For a sphere, area of surface is 4πR2, where R is the radius.So Luminosity α R2 x T4
9 Clicker Question: Compared to blue light, red light travels: A: faster B: slowerC: at the same speed
10 Clicker Question: A star much colder than the sun would appear: A: red B: yellowC: blueD: smallerE: larger
12 The Doppler EffectApplies to all waves – not just radiation. The frequency or wavelength of a wave depends on the relative motion of the source and the observer.
13 Applies to all kinds of waves, not just radiation. The Doppler EffectApplies to all kinds of waves, not just radiation.at restvelocity v1velocity v1velocity v2you encounter more wavecrests per second => higher frequency!velocity v1velocity v3fewer wavecrests per second => lower frequency!
14 Waves bend when they pass through material of different densities. Things that waves do1. RefractionWaves bend when they pass through material of different densities.airwaterswimming poolprismglassairair
15 2. DiffractionWaves bend when they go through a narrow gap or around a corner.
16 Clicker Question:If a star is moving rapidly towards Earth then its spectrum will be:A: the same as if it were at restB: shifted to the blueC: shifted to the redD: much brighter than if it were at restE: much fainter than if it were at rest
17 Spectroscopy and Atoms How do we know:- Physical states of stars, e.g. temperature, density.- Chemical make-up and ages of stars, galaxies- Masses and orbits of stars, galaxies, extrasolar planetsexpansion of universe, acceleration of universe.All rely on taking and understanding spectra: spreading out radiationby wavelength.
18 Types of Spectra and Kirchhoff's (1859) Laws 1. "Continuous" spectrum - radiation over a broad range of wavelengths (light: bright at every color). Produced by a hot opaque solid, liquid, or dense gas.2. "Emission line" spectrum - bright at specific wavelengths only. Produced by a transparent hot gas.3. Continuous spectrum with "absorption lines": bright over a broad range of wavelengths with a few dark lines. Produced by a transparent cool gas absorbing light from a continuous spectrum source.
19 The pattern of lines is a fingerprint of the element (e. g The pattern of lines is a fingerprint of the element (e.g. hydrogen, neon) in the gas.For a given element, emission and absorption lines occur at the same wavelengths.Sodium
20 The Particle Nature of Light On microscopic scales (scale of atoms), light travels as individual packets of energy, called photons. (Einstein 1905).cphoton energy is proportional toradiation frequency:E (or E )example: ultraviolet photons are more harmful than visible photons.1
21 The Nature of Atoms The Bohr model of the Hydrogen atom (1913): electron__++proton"ground state"an "excited state"Ground state is the lowest energy state. Atom must gain energy to move to an excited state. It must absorb a photon or collide with another atom.
22 But, only certain energies (or orbits) are allowed: ___a few energy levels of H atom+The atom can only absorb photons with exactly the right energy to boost the electron to one of its higher levels.(photon energy α frequency)
23 When an atom absorbs a photon, it moves to a higher energy state briefly When it jumps back to lower energy state, it emits a photon - in a random direction
24 Other elements Helium Carbon neutron protonAtoms have equal positive and negative charge. Each element has its own allowed energy levels and thus its own spectrum. Number of protons defines element. Isotopes of element have different number of neutrons.
25 IonizationHydrogen_+Energetic UV Photon_Helium++Energetic UV Photon_Atom"Ion"Two atoms colliding can also lead to ionization. The hotter the gas, the more ionized it gets.
26 So why do stars have absorption line spectra? Simple case: let’s say these atoms can only absorb green photons. Get dark absorption line at green part of spectrum............“atmosphere” (thousandsof K) has atoms and ionswith bound electronshot (millions of K), dense interiorhas blackbody spectrum,gas fully ionized
27 Stellar SpectraSpectra of stars differ mainly due to atmospheric temperature (composition differences also important).“hot” star“cool” star
28 Why emission lines?hot cloud of gas......- Collisions excite atoms: an electron moves to a higher energy level- Then electron drops back to lower level- Photons at specific frequencies emitted.
29 We've used spectra to find planets around other stars.
30 Star wobbling due to gravity of planet causes small Doppler shift of its absorption lines. Amount of shift depends on velocity of wobble. Also know period of wobble. This is enough to constrain the mass and orbit of the planet.
31 Now more than 300 extrasolar planets known Now more than 300 extrasolar planets known. Here are the first few discovered.
32 So why absorption lines? .....cloud of gas......The green photons (say) get absorbed by the atoms. They are emitted again in random directions. Photons of other wavelengths go through. Get dark absorption line at green part of spectrum.
33 Molecules Two or more atoms joined together. They occur in atmospheres of cooler stars, cold clouds of gas, planets.ExamplesH2 = H + HCO = C + OCO2 = C + O + ONH3 = N + H + H + H (ammonia)CH4 = C + H + H + H + H (methane)They have- electron energy levels (like atoms)- rotational energy levels- vibrational energy levels
35 Types of Spectra1. "Continuous" spectrum - radiation over a broad range of wavelengths(light: bright at every color).2. "Emission line" spectrum - bright at specific wavelengths only.3. Continuous spectrum with "absorption lines": bright over a broad range of wavelengths with a few dark lines.
36 Kirchhoff's Laws (1859)1. A hot, opaque solid, liquid or dense gas produces a continuous spectrum.2. A transparent hot gas produces an emission line spectrum.3. A transparent, cool gas absorbs wavelengths from a continuous spectrum, producing an absorption line spectrum.