Perspective on the Universe Phys 1830 Lecture 6

1. Perspective on the UniversePhys 1830 Lecture 6 summary Text Recall column Text • 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 next Friday -- any particular targets? • Check the Hubble Legacy Archive Change of password If you didn’t register your iclicker in class, email Dr. English your name, iclicker # & student # AND course.

2. Phys 1830 Lecture 6 summary Text Recall column Text • This Class and the Next: • Wave & Particle models of light • Colour & Temperature • Interaction of Light with Matter

3. What are we seeing here? • Gravity! Light behaving according to the General Theory of Relativity.

4. 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

5. The Electromagnetic Spectrum summary Text (EM) Recall column Text

6. summary Text Recall column Text • 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!

7. Frequency & Wavelength summary Text Recall column Text The speeds of both waves are the same This wavelength is twice as long

8. The Electromagnetic Wave summary Text Recall column Text • changing electric field generates a magnetic field • changing magnetic field generates an electric field

9. Electromagnetic Wave summary Text Recall column Text • oscillations occurring perpendicular to the direction of energy transfer • oscillating electric & magnetic fields

10. Velocity of a Wave summary Text Recall column Text 5 • 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)

11. 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

12. 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.

13. Visible Light summary Text Recall column Text nanometer = nm = 10-9 m Ångströms = Å = 10-10 m • 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

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

15. The Nature of Light 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

16. 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”

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

18. Thermal Radiation summary Text Recall column Text • 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)

19. Thermal Radiation summary Text Recall column Text • high thermal energy produces high energy photons! • high energy photons = X-Rays, Gamma 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

20. Temperature Scales summary Text Recall column Text

21. Blackbody Radiation summary Text Recall column Text • 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.

22. Blackbody Radiation summary Text Recall column Text • 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

23. Blackbody Radiation summary Recall column Explained using particle theory of light photons of energy E=hν Intensity 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. Wavelength 

24. 20,000° K 10,000° K 5000° K Intensity 2000° K 1000° K 500° K X-Ray Ultraviolet Visible Infrared Microwave Radio Wavelength (nm) Blackbody Radiation Curves for Different Temperatures summary Text Recall column

25. Phys 1830: Lecture 6 • Next class: • black body radiation continued. • Kirchhoff’s Laws • Spectra • How the interaction of light and matter produce spectra.