Read BEFORE coming to class: Lecture 8 Phys 1810 Read BEFORE coming to class: Electromagnetic Radiation 3.1 to 3.4 Energy Thermal Radiation Box 3-2 Flux and Luminosity (L equation in Box 17-2) Spectra 4.1, 4.2 Kirkhhoff’s Laws Radio Emission 18.4 Doppler shift: 3.5, Box 3-3, 4.5 Telescopes 5.2, 5.3, “seeing” in 5.4, 5.5-5.7 Password! Given out in Class, not email. TODAY! Office hour 3pm Allen 514 Not D2L, not JUMP, not Mastering Astronomy To do practice questions for test/exam, the textbook online code is required. The class lecture website is http://www.physics.umanitoba.ca/~english/2014fallphys1810/

[EM] Time to snap fingers 1/10 sec => light travels ¾ of distance around the Earth.

Note that the waves are perpendicular.

Electromagnetic Wave summary Text Recall column Text See another good animation: http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=35 oscillations occurring perpendicular to the direction of energy transfer oscillating electric & magnetic fields

 a B field accompanies a changing E field. Thus a vibrating charged particle in a star create EM waves in its own EM field and these waves propagate through space. Objects like magnetized needle in compass, Earth’s magnetic poles. B is conventionally written with an arrow on top.

Hole in wall

Hole in the Wall Expected for particles Observed

What happens if you send photons one at a time through a double slit? Check the class website for videos! Double Slit  Interference Pattern http://www.olympusmicro.com/primer/java/doubleslitwavefronts/index.html What happens if you send photons one at a time through a double slit? You can do a version of the double slit at home using a laser pointer and covering a small comb with tape, except for 2 slits. Some light sources emit 1 photon at a time, so this experiment has been done. Light is both a wave and a particle ! Would you get only 2 strips as if the photons were “baseballs” ? https://www.youtube.com/watch?v=MbLzh1Y9POQ Demonstrates the DUAL NATURE of light.

Particle Description  Photons

Photon Energy (E) h== Planck constant. but So Higher frequencies have higher energies Photon energy. h is a constant that would be supplied on a test or exam. How does the speed of radio waves compare to the speed of visible light? They both travel at the same speed.

Note: Visible light isn’t special Relation between wavelength & frequency Atmospheric opacity & transparency. You may want to write down what I say about each range and transparency.

Thermal Radiation “heat” most familiar kind of radiation summary Text Recall column Text “heat” most familiar kind of radiation caused by random motions of atoms & molecules a lot of energy available large amount of motion (high temperature) There are other forms of radiation, as we will see. T== temperature

Blackbody Radiation summary Text Recall column Text blackbody (b.b.) radiation is thermal radiation emitted a blackbody blackbody == “perfect absorber” & re-emits radiation in all directions! (doesn’t scatter) no “perfect” blackbody but close: some ovens stars (sun) cosmic background radiation Stars don’t have a surface to scatter light for example – they absorb and re-radiate.

Temperature Scales We use the Kelvin scale for astronomical phenomena. summary Text Recall column Text Zero points are all differentAll objects above absolute zero have some random (thermal) motions!At absolute zero, all random motions stop! We use the Kelvin scale for astronomical phenomena.

Blackbody Radiation b.b.s emit across a range of λ summary Text Recall column Text b.b.s emit across a range of λ but intensity not the same at all λ Temperature (T) of b.b. determines intensity of radiation & the peak λ

Blackbody Radiation b.b. radiation depends only on its T. summary Recall column Intensity Intensity Wavelength  Wavelength increases to the right; frequency increases to the left. High frequency = high energy = bluer colour. Low frequency = low energy = redder colours. b.b. radiation depends only on its T. The intensity changes at different wavelengths. (Graph of 1 object at 1 specific T.)

Blackbody Radiation: b.b. curve summary Recall column Explained using particle theory of light photons of energy Intensity Intensity Wavelength  Frequency  Wavelength increases to the right; frequency increases to the left. High frequency = high energy = bluer colour. Low frequency = low energy = redder colours. E == energy h == is a constant (given on tests) nu == frequency lambda == wavelength which end of the spectrum has higher energy? which lower? b.b. radiation depends only on its T. X-axis Book uses increasing frequency (nu). Most astronomers use increasing wavelength (lambda), so we’ll use this.

Blackbody Radiation: Wien’s Law summary Wavelength  Recall column Intensity Peak Intensity Relates the temperature of b.b. to its maximum (i.e. peak) emission. K== degrees Kelvin m== metres

Blackbody Radiation Curves for Different Temperatures summary Text Recall column Wavelength (nm) 500° K 1000° K 2000° K 5000° K 10,000° K 20,000° K X-Ray Ultraviolet Visible Infrared Microwave Radio Intensity Each curve is called a spectrum, the plural is called spectra. A spectrum shows how intensity of radiation changes with wavelength. The position of the peak shifts towards shorter wavelengths (higher energy) as the temperature increases. Intensity increases dramatically as the temperature increases.

Example using Wien’s Law: .

E. g. If is at short wavelengths for a b. b E.g. If is at short wavelengths for a b.b., then its T is higher & the object’s emission is towards blue end of EM spectrum.

summary Text Recall column Intensity Short Long Wavelength Star A Star B Star C Star D Star E A plot of blackbody spectra of five different stars is shown in the figure. Based on these spectra, the star with lowest T is Star E. T==temperature

What colour does this star have? summary Recall column Red The colour stripes give you an idea of the visual range for electromagnetic radiation. The curve represents the spectrum of the star.

(greenish) Yellow-white summary Recall column (greenish) Yellow-white

summary Recall column Blue

what is hot & what is not? The hottest stars in this image appear: summary Recall column The hottest stars in this image appear: Blueish Reddish Relate this to public outreach images.

Contrast with everyday experience! summary Recall column Note that the public audience reads “blue” as cold. Artists call blues “cool” and yellow + reds “warm”.

Thermal Radiation from Astronomical Objects On supplemental page. summary Examples of Blackbodies and their Temperatures. Recall column Thermal Radiation from Astronomical Objects Object Temperature (K) Peak Wavelength Electromagnetic Region Cosmic Background 3 1 mm Microwave Molecular Cloud (stellar cores) 10 300 μm Microwave/Infrared Humans 310 9.7 μm Infrared Incandescent Light Bulb 3000 1 μm or 10,000 Å Infrared/Visible Sun 6000 5000 Å Visible Hot Star 30,000 1000 Å Ultraviolet Intra-Cluster Gas 100,000,000 0.3 Å X-Ray Cosmic Microwave Background radiation is the best physical example of a blackbody. Intra-cluster gas is the gas between galaxies within a cluster of galaxies. (Note the value for the sun is rounded up.)

Objects and Peak of Emission Dense, spherical clouds: radio and Far-IR Globule of dust: IR Sun: visible IR spitzer and UV galex. White dwarf star/planetary nebula: UV

Flux related to temperature (T) Stefan-Boltzmann law for b.b: Relates T to the total amount of energy (E) that the b.b. emits at all wavelengths.

Note direct proportionality. energy/sec is a “rate” since per sec Note direct proportionality. energy/sec is a “rate” since per sec. Physicists use m^2 while astronomers use cm^2. BALLOON EXAMPLE

Balloon & Surface Area 2 stars with the same T but different surface area. Total energy output over the whole sphere of larger object is larger. A match and a log may have the same T but total amount of radiation (total amount of heat) emitted is different.

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Stars: Their Characteristics Luminosity (L): The total energy radiated per second, at all wavelengths. L = surface area * flux Surface area of a sphere is LUMINOSITY IS AN INTRINSIC PROPERTY! T== Surface Temperature  Luminosity is proportional to the radius squared times surface temperature to the 4th power.

Stars: Why Temperature is useful. Notice that if we know the temperature of a star, then if we know the radius, we can calculate the luminosity. Alternatively, if we know the temperature and the luminosity we can determine the radius. 

The Interaction of light and matter. summary Recall column Photons (γ == gamma) Individual packet of EM energy that makes up EM radiation γ & matter interact creating spectra. Spectra used to assess T (blackbody curve type spectrum) processes that produce light or absorb it (i.e. what is going on) (Animation)

Kirchhoff’s Laws Spectra 3 empirical laws summary Recall column Kirchhoff’s Laws 3 empirical laws Hot opaque body -> continuous spectrum Cooler transparent gas between source & observer -> absorption line spectrum Diffuse, transparent gas -> emission line spectrum  means “gives” Astronomical objects have spectral “finger prints”. Also called “Continuum emission” can be produced by a hot dense gas. (Like a light bulb – a rainbow of colours.) If we plot this as intensity versus wavelength this produces a blackbody curve. c) Emission lines are produced by diffuse (low density) gas. (Like a neon sign.) b) Place the diffuse gas in front of the hot, gas and absorption-lines are created. There is the continuum (rainbow) with dark absorption lines where the emission lines would be. (show animation)

Spectra This kind of spectrum (continuum) is caused by summary Recall column This kind of spectrum (continuum) is caused by Hot, low density gas Hot, dense blackbody Cooler transparent gas

Continuum rainbow-like spectrum Dark line absorption spectrum Spectra Our sun and other stars have an atmosphere. Imagine that you are in a spaceship far above the Earth’s atmosphere. Which of the following spectra would you observe when analyzing sunlight? Continuum rainbow-like spectrum Dark line absorption spectrum Bright line emission spectrum Discuss with your neighbours. Answer b.

Spectral Finger Prints Solar Spectrum Note emission lines for lab spectrum of iron are at same λs of absorption lines of iron in  Can use line spectra to determine chemical elements in object.

Interaction of Light and Matter: How are line spectra created? γs of light interact with atoms & molecules. Atoms consist of: Electrons (negative charge) == e- Nuclei (balance charge of e-) Protons (positive) Neutrons (neutral) Molecules: group of 2 or more atoms. This happens to be a helium atom. protons, +ve, 2000x e- mass

Interaction of Light and Matter Hydrogen == H: simplest atom. 1 e- & 1 proton. Classical picture: e- in an orbit. Contemporary picture: e- as a cloud. Orbits are really energy levels. E == energy

Interaction of Light and Matter Hydrogen Atom Energy Levels Every chemical element has its own specific set of E levels. Each E level is associated with a λ. Recall energy is E=h * frequency and c = wavelength * frequency. So substituting for frequency we can see that E is associated with wavelength. E propto 1/lambda

Interaction of Light and Matter Creating spectral lines at visible wavelengths specific (quantized) E levels. Level with lowest E is ground state. How does e- get excited? By interactions between γs & matter.

Interaction of Light and Matter: Creating spectral lines at visible wavelengths The e- can shift between E levels by absorption & emission of γs.

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