PTYS 214 – Spring 2011
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/waveParts.htm
Announcements
Quiz #3
Total Students: 24
Class Average: 2.83
Low: 1
High: 4
One quiz grade will be dropped from the final grade mean
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Homework #3
Total Students: 23
Class Average: 7.91
Low: 4
High: 10
One homework grade will be dropped from the final grade mean
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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
Solar energyfrom hydrogen fusion Photosynthesis
Solar Radiation
Climate
Energy transported by electromagnetic waves
What is Solar Radiation?
Electromagnetic waves are not limited to visible light (sunlight)
X-rays, Radio waves, Microwaves are all electromagnetic waves
Electric field: produced by stationary charges Magnetic field: produced by moving charges (currents) Changing magnetic fields produce electric fields
Electromagnetic 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, :
Properties of Waves
Distance
(Hertz) P
1
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 /
λνP
λc
Properties of Waves
Acts like particles: absorbed by atoms as discrete energy packages,
photons
Dual Nature of Electromagnetic Radiation
Acts like waves: interference, refraction
hc
En
Energy of a photon is proportional to frequency,and inversely proportional to wavelength
where h = Planck’s constant
Packets of Electromagnetic Radiation Photons
(“particles” with no mass)
hchE
Electromagnetic Spectrum
(m)
ultravioletvisiblelightinfraredmicrowaves X-rays
HighEnergy
LowEnergy
( = “micro” = 10−6)
0.7 to 0.4 m
1000 100 10 1 0.1 0.01
(1/sec)
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
Solar Spectrum
Wavelength (m)
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
Visible Light (VIS)
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
Has the right energy to break molecular bonds apart
Three Categories: Name Wavelength(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
UV Radiation
UV-B concern
Overexposure can cause:
Animals: skin cancer, cataracts, suppressed immune system ….
Plants: photosynthesis inhibition, leaf expansion, plant growth …
DNA absorb UV-B
1) UVB photons excite DNA
2) Adjacent C bases form a dimer
3) DNA polymerase “reads” CC dimer as AA
4) New strand would get TT instead of GG
C-T Mutation
Too many of these mutation within the DNA and the damage could lead to the inability of the cell to carry out normal functions
The UV Index
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 colorHere white and yellow correspond to hot regions, blue and green to cool region
Infrared (IR) radiation
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
Thermal Energy
Scale melting point of ice
boiling point of water
Fahrenheit - oF 32 212
Celsius - oC 0 100
Kelvin - K 273 373
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
Temperature Scales
Fahrenheit , Celsius and KelvinoC + 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°C58 + 273 = 331 K
Lowest: Vostok, Antarctica: -89.2°C (in 1983)(-89 x 1.8) + 32 = -128.2°F
Temperature Scales:
Blackbody RadiationThe Sun emits like a blackbody, a body that is both a perfect emitter and absorber
1) All objects emit radiant energy (electromagnetic waves)
2) Hotter objects emit more energy per unit area than colder objects (Stefan-Boltzmann’s Law)
3) The hotter the object, the shorter the wavelength () of maximum emitted energy (Wien’s Law)
Basic Laws of Radiation
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: 5.6710-8 W/(m2K4)
4σTF
T (K) F (W/m2)
Sun 6000 7×107
Body #2 T2=2×T1 ?
Wien’s LawObjects of different temperature emit spectra that peak at different wavelengths:
T(K)
3000m)(max
An object’s color defines its temperature
Cooler objects emit most of their radiation at longer wavelengths
The Sun’s surface temperature is about 6000K
What is the wavelength of maximum energy emission?
Sun’s Peak Wavelength
μm 0.5λmax
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
Why does the sun look yellow?
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 atmosphereSome radiation is absorbed by atmospheric gases
2. Earth is sphericalThe same solar beam would “cover” different areas in the equatorial and polar regions
Earth has an atmosphereAtmospheric gases absorb radiation at particular wavelengths
Earth is spherical
As the solar radiation reaches the surface at increasing angles, it is going to be distributed over a larger area
More diffuse
Surface area receiving insolation
More concentrated
More diffuse
Oblique
Oblique
Direct
S (local) = S0 × cos(Latitude)
cos(0)=1 cos(30)=0.866 cos(60)=0.5
Latitude (Tucson)= 32Latitude (Copenhagen, Denmark) ~ 56
S (Tucson) = ? S (Copenhagen) = ?
S (local) S0 × cos(Latitude)
cos(0)=1 cos(30)=0.866 cos(60)=0.5
Latitude (Tucson)= 32Latitude (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)
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