PTYS 214 – Spring 2011

1. Announcements • Homework #4 is posted on the class website DUE on Tuesday, Feb. 15 • Today:last day to drop a class and not have it on your academic record • Class website: http://www.lpl.arizona.edu/undergrad/classes/spring2011/Pierazzo_214/ • Useful Reading: class website  “Reading Material” http://en.wikipedia.org/wiki/Sunlight http://www.sciencedaily.com/releases/2009/08/090827141349.htm http://id.mind.net/~zona/mstm/physics/waves/partsOfAWave/waveP arts.htm PTYS 214 – Spring 2011

2. Quiz #3 • Total Students: 24 • Class Average: 2.83 • Low: 1 • High: 4 One quiz grade will be dropped from the final grade mean

3. Homework #3 • Total Students: 23 • Class Average: 7.91 • Low: 4 • High: 10 One homework grade will be dropped from the final grade mean

4. The Sun’s Fate • In about 5 billion years the hydrogen in the core will not be enough to continue fusion (He accumulates) • Core will shrink – pp nuclear fusion occurs outside the core • The outer layer of the Sun would expand, eventually beyond the Earth’s orbit – Red Giant phase • It is likely that Earth will be destroyed

5. Solar energy from hydrogen fusion Photosynthesis Solar Radiation Climate

6. What is Solar Radiation? Energy transported by electromagnetic waves Electromagnetic waves are not limited to visible light (sunlight) X-rays, Radio waves, Microwaves are all electromagnetic waves

7. Electromagnetic Waves • Electric field: produced by stationary charges • Magnetic field: produced by moving charges (currents) • Changing magnetic fields produce electric fields

8. Properties of Waves Wavelength Wavelength: peak to peak distance,  Period:time that it takes for a wave to oscillate from peak to peak, P Frequency: number of waves that pass a point per unit time, : Distance

9. Properties of Waves Velocity: speed at which the shape of the wave is moving: All electromagnetic waves travel at the same speed, the speed of light: 300,000 km/s Hence,  = c /  and  = c×P =c / 

10. Dual Nature of Electromagnetic Radiation Acts like particles: absorbed by atoms as discrete energy packages, photons Acts like waves: interference, refraction

11. Packets of Electromagnetic Radiation Photons (“particles” with no mass) Energy of a photon is proportional to frequency, and inversely proportional to wavelength whereh= Planck’s constant

12. 1000 100 10 1 0.1 0.01 Electromagnetic Spectrum visible light infrared ultraviolet X-rays microwaves 0.7 to 0.4 m Low Energy High Energy  (m)  (1/sec) ( = “micro” = 10−6)

13. Solar Spectrum The sun emits radiation at all wavelengths Most of its energy is in the IR-VIS-UV parts of the spectrum ~44% of the energy is in the visible ~37% in the near-IR ~8% in the UV Wavelength (m)

14. Visible Light (VIS) 0.7 to 0.4 m Our eyes are sensitive to this region of the spectrum Photosynthesis mostly uses visible radiation Red-Orange-Yellow-Green-Blue-Indigo-Violet

15. Why are plants green? • Green plants effectively absorb • violet, blue and red radiation Green wavelengths are not absorbed effectively, making plants look green • Red algae absorb blue-green radiation, causing algae to look red

16. UV Radiation Has the right energy to break molecular bonds apart Three Categories: NameWavelength(s)Biological Effect UV-A > 320 nm (0.32 m) Harmful(?) UV-B 280-320 nm Harmful, partially blocked by O3 UV-C < 280 nm Very harmful, but blocked by O3 and O2

17. UV-B concern Overexposure can cause: Animals:skin cancer, cataracts, suppressed immune system …. Plants:photosynthesis inhibition, leaf expansion, plant growth … DNA absorb UV-B

18. C-T Mutation • UVB photons excite DNA • Adjacent C bases form a dimer • DNA polymerase “reads” CC dimer as AA • 4) New strand would get TT instead of GG Too many of these mutation within the DNA and the damage could lead to the inability of the cell to carry out normal functions

19. The UV Index

20. Infrared (IR) radiation 0.7 μm to ~1 mm We can’t see IR, but we can feel it as thermal heat Lower energy than visible light IR image of a human hand displayed in false color Here white and yellow correspond to hot regions, blue and green to cool region

21. Thermal Energy • All matter is composed of atoms or molecules, which are in constant motion Thermal energy is the total kinetic (motion) energy of molecules or atoms in a substance Heating causes atoms to move faster, causing an increase in thermal energy Temperature is a measure of thermal energy, the average chaotic motion of atoms or molecules

22. Temperature Scales Relative size of a degree F vs. a degree C Compare the number of degrees between freezing and boiling: 100K = 100oC = 180oF 1 K = 1oC = 1.8oF

23. Temperature Scales: Fahrenheit , Celsius and Kelvin oC+ 273 = K (oC x 1.8) + 32 = oF (oF - 32) / 1.8 = oC Example: Extreme recorded temperature on Earth Highest: El’Azizia, Libya: 136.4°F (in 1922) (136.4 - 32) / 1.8 =58°C 58 + 273 =331 K Lowest: Vostok, Antarctica: -89.2°C (in 1983) (-89 x 1.8) + 32 = -128.2°F

24. Blackbody Radiation The Sun emits like a blackbody, a body that is both a perfect emitter and absorber

25. Basic Laws of Radiation • All objects emit radiant energy (electromagnetic waves) • Hotter objects emit more energy per unit area than colder objects (Stefan-Boltzmann’s Law) • The hotter the object, the shorter the wavelength () of maximum emitted energy (Wien’s Law)

26. Stefan-Boltzmann’s Law The total energy emitted by a blackbody at all wavelengths is directly proportional to the 4th power of its temperature F = energy emitted per m2 σ = constant: 5.6710-8 W/(m2K4)

27. Wien’s Law Objects of different temperature emit spectra that peak at different wavelengths: An object’s color defines its temperature Cooler objects emit most of their radiation at longer wavelengths

28. Sun’s Peak Wavelength The Sun’s surface temperature is about 6000K What is the wavelength of maximum energy emission?

29. Why does the sun look yellow? Rayleigh scattering in the Earth’s atmosphere removes blue light from the solar radiation, so from the Earth’s surface the Sun appears yellow, even though its radiation peaks in the green

30. Solar Energy at the Surface Solar Constant is the amount of solar radiation per unit area, measured at the outer surface of Earth's atmosphere in a plane perpendicular to the rays But! 1. Earth has an atmosphere Some radiation is absorbed by atmospheric gases 2.Earth is spherical The same solar beam would “cover” different areas in the equatorial and polar regions

31. Earth has an atmosphere Atmospheric gases absorb radiation at particular wavelengths

32. Earth is spherical More diffuse Oblique Surface area receiving insolation Sun’s rays arrive parallel at the Earth More concentrated Direct Oblique More diffuse As the solar radiation reaches the surface at increasing angles, it is going to be distributed over a larger area

33. S (local) = S0 × cos(Latitude) cos(0)=1 cos(30)=0.866 cos(60)=0.5 Latitude (Tucson)= 32 Latitude (Copenhagen, Denmark) ~ 56 S (Tucson) = ? S (Copenhagen) = ?

34. S (local)  S0 × cos(Latitude) cos(0)=1 cos(30)=0.866 cos(60)=0.5 Latitude (Tucson)= 32 Latitude (Copenhagen, Denmark) ~ 56 S (Tucson) = 85% of equatorial S (Copenhagen) = 56% of equatorial Polar regions always get less solar flux than equatorial regions (that’s why polar regions are colder)

35. Summary • Solar flux decreases as radiation spreads out away from the Sun • Planets are exposed to some small amount of the total solar radiation • Some portion of that radiation can be used for photosynthesis • Other biota can eat energy-rich organic molecules from photo-autotrophs or each other