Perspective on the Universe Phys 1830 Lecture 7 summary Text Recall column Previous Class: Order of Magnitude example Distances: parallax out to 200 pc parsec Inverse Square Brightness Law for distant stars This Class electromagnetic radiation Next Class the interaction between matter and light Black & White workshop coming up! Text Considered the vastness of the universe – converting kpc to ly we found that a signal from MW centre takes at least 24,000 years! Hard for civilizations to communicate. Change of password If you didn’t register your iclicker in class, email Dr. English your name, iclicker # & student # AND course.

Material tested is from quiz to quiz. Quizzes are based on material from the previous Friday and the current Monday and Wednesday.

“Student Accessibility Services requires volunteer note-takers for this class. If you would like to volunteer, and receive a reference letter for your time, please login to JUMP and click on the Student Accessibility Services link on the right hand side of the page. Volunteering is now easier than ever, you can upload notes directly to JUMP with the SAS scheduler. Thank you.”

Gravity! Light behaving according to the General Theory of Relativity. What are we seeing here? We even use light to study gravity. Arcs are distortions caused by a foreground gravitational lens consisting of a cluster of galaxies. Abell Cluster 2188 imaged by HST. Gravity! Light behaving according to the General Theory of Relativity.

Our Universe summary Text Recall column Abell 1689 Text Galaxy Cluster and Lensed Galaxy 2.2 billion light-years (to lensing cluster) 12.8 billion light-years (to lensed galaxy) information about our universe comes from light electromagnetic radiation very distant objects (basically anything beyond our Solar System) are inaccessible for direct study all the information we have about these objects comes from their radiation

The Electromagnetic Spectrum summary Text (EM) Recall column Text Electromagnetic radiation spans a range larger than what our eye detects. The arrow indicates the direction wavelength increases and energy decreases. Regions of the spectrum are categorized as Gamma Rays (high energy), X-rays, visible/optical light, Infra-red radiation, Microwaves and Radio waves.

the wave propagates through space at the speed of light = 300 000 km/s summary Text Recall column Text Imagine placing your hand in a pond of water and creating ripples. These are waves with the features in the diagram. EM Radiation is a wave the wave propagates through space at the speed of light = 300 000 km/s all EM Radiation: Radio through Visible through X-Rays propagates at the same speed!

Frequency & Wavelength summary Text Recall column Text The speeds of both waves are the same The upper wave has a shorter wavelength and a higher frequency The lower wave has a longer wavelength and a lower frequency High frequency: more waves pass per second! This wavelength is twice as long

The Electromagnetic Wave summary Text Recall column Text demos. changing electric field generates a magnetic field changing magnetic field generates an electric field Demos - take notes

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

Velocity of a Wave velocity = wavelength (λ) * frequency (ν) summary Text Recall column Text velocity = wavelength (λ) * frequency (ν) Velocity of an Electromagnetic Wave? Speed of light! Abbreviated: c c = λν c = 3 * 108 m/s (= 3 * 10 km/s) λ has units of nanometers (10-9 m) or sometimes Ångströms = Å = 10-10 m ν has units of Hertz = s-1 (or per second) 5 wavelength – lambda frequency – nu Recall that “*” = “times” (multiplication)

Wavelength & Frequency summary Text Recall column Text because the speed of light is a constant (c = 3 x 108 m/s) if we know the wavelength, we can calculate the frequency! What is the frequency of 550 nm green light? (re-arrange equation) ν = c / λ ν = 3 * 108 m/s / 550 x 10-9 m ν ~ 5 * 1014 /s ν ~ 5 * 1014 Hz What frequencies do radios operate at? How do they compare with visible light frequencies?

iClicker Question summary Text Recall column Text How does the speed of radio waves compare to the speed of visible light? Radio waves are much slower. They both travel at the same speed. Radio waves are much faster.

nanometer = nm = 10-9 m Ångströms = Å = 10-10 m Visible Light summary Text Recall column Text nanometer = nm = 10-9 m Ångströms = Å = 10-10 m phenomenon known as dispersion pass ordinary sunlight through a prism and you will see a rainbow prism refracts (bends) the light blue light refracts more than red light, so the colours separate out

In nature - the wondrous rainbow summary Text Recall column Text Drops of water suspended in the air.

The Nature of Light Two Theories Historically Waves (Huygens) summary Text Recall column Text Two Theories Historically Waves (Huygens) Particles (Newton) both used in modern times Wave Theory Phenomena diffraction interference polarization Particle Theory Phenomena photons spectral lines black bodies Light is both a wave and a particle – it has a DUAL nature. diffraction – how light spreads out interference – how waves add together to form a new wave pattern polarization – how waves are oriented ... e.g. oriented so that they are travelling in 1 direction via polaroid glasses photons – packets of energy ... One can think of photons as packets of waves. spectral lines – e.g. emission from one element, like neon black bodies – emit photons at all wavelengths... see later in this lecture

Theoretical Predictions summary Text side on view face on view Recall column if round hole in wall Text e.g. “baseballs” e.g. “water waves”

What happens if you send photons one at a time through a double slit? Double Slit  Interference Pattern 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 ! http://www.olympusmicro.com/primer/java/doubleslitwavefronts/index.html What happens if you send photons one at a time through a double slit? Would you get only 2 strips as if the photons were “baseballs” ? http://www.youtube.com/watchv=MbLzh1Y9POQ Demonstrates the DUAL NATURE of light.

Thermal Radiation summary Text Recall column this is the most familiar kind of radiation (there are other kinds too!) thermal radiation is caused by the random motions of atoms and the electrons in the atoms if there is a lot of energy available, there will be a large amount of motion (indicated by a high temperature) Text

Thermal Radiation high thermal energy produces high energy photons! summary Text Recall column high thermal energy produces high energy photons! high energy photons = Gamma Rays, X- Rays, low energy photons = Radio waves thermal motion is random motion motions can also be due to magnetic fields or nuclear processes - this radiation is not thermal Text

Temperature Scales We use the Kelvin scale for astronomical phenomena. summary Text Recall column Text Zero points are all different. All 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 summary Text Recall column blackbody radiation is thermal radiation emitted by an object (a blackbody) a blackbody is an object that is a “perfect absorber” (it absorbs all the radiation that hits it) - it then re-emits that radiation in all directions! there is no “perfect” blackbody in practice, but the Sun (and all other stars) come close! Even better is the Cosmic Microwave Background Radiation. Text

Blackbody Radiation summary Text Recall column all objects are blackbodies to some degree (everything will absorb some radiation, and everything re-emits this radiation with varying degrees of efficiency) blackbodies don’t just emit at one wavelength, they always emit across a range of wavelengths but, the intensity of the radiation will not be the same at all wavelengths the temperature of the blackbody determines the intensity of the radiation, and the peak wavelength Text

Blackbody Radiation Explained using particle theory of light summary Recall column Explained using particle theory of light photons of energy E=hν Intensity Got to here: 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 nu == frequency Wavelength  The radiation emitted by a blackbody depends only on its temperature. This graph shows how the intensity of the radiation changes at different wavelengths. This graph is for an object at one specific temperature.

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.

Thermal Radiation from Astronomical Objects 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 Infrared Humans 310 9.7 μm 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.

Match these objects to peak temperatures in the previous table. Objects and Peak of Emission Dense, spherical clouds: radio and Far-IR. Exercise for at home: Match these objects to peak temperatures in the previous table. Globule of dust: IR. IR spitzer and UV galex. Sun: visible White dwarf star/planetary nebula: UV

Star A Star B Star C Star D Star E summary iClicker Question Text Recall column Intensity Short Long Wavelength Star A Star B Star C Star D Star E A plot of the blackbody spectra of five different stars is shown in the figure. Based on these spectra, which of the stars has the lowest temperature? Star A Star B Star C Star D Star E

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.

summary Recall column Yellow

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”.

The Interaction of light and matter. summary Recall column Photons and matter interact creating spectra. spectra can be used to assess temperature (blackbody curve type spectrum) processes that produce light or absorb it (i.e. what is going on)

Kirchhoff’s Laws Spectra 3 empirical laws summary Recall column  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) 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

Next class: Kirchhoff’s Laws Spectra Phys 1830: Lecture 7 Next class: Kirchhoff’s Laws Spectra How the interaction of light and matter produce spectra.