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