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Today's contents
• Review of Electromagnetic and Thermal Radiation
• Time and Distance
Nature of Light
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Radiant energy (방사에너지)
Interacts with matter via either:
1.
Emission: Light is released
2.
Absorption: Light is captured
3.
Reflection (반사), Refraction (굴절), Diffraction (회절)
Moves through a vacuum with a constant speed, c=3108 m/s
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Exhibits Wave (파도,물결) Nature
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Exhibits Particle (입자) Nature
Visible light is the light that human eyes are sensitive to. Different
colors correspond to electromagnetic waves having different
wavelengths
Wavelength or 
- Violet
- Blue
- Green
- Yellow
- Red
400 nm
480 nm
500 nm
580 nm
700 nm
White light is then made up of
equal numbers of photons of all
wavelengths.
Monochromatic ==> 단색의
(Energy) = hv = hc/
Interaction between light and atoms
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The electrons can only occupy discrete (이산의) energy levels within Atoms,
Ions, and Molecules.
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In order for an electron to make a transition from one level to the other it must
change its energy by an amount equal to the difference in the levels.
Absorption and Emission (흡수 와 방출)
Excited state
Ground state
회전
진동(떨림)
Molecules may also absorb and emit light of specific energies that
correspond to discrete states of rotation and vibration.
Doppler Effect (도플러 효과)
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Similarly, objects in motion compress the light waves in front of them making
them appear bluer, while the light waves behind are stretched out and appear
redder.
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Amount of shift of wavelength is proportional to the component of velocity along
the line of sight.
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Can calculate the velocity of motion along the line of sight:
/0 = v/c
where c is the speed of light, 0 is the wavelength of the light as seen at rest, and
 is the measured change in wavelength.
Note: Sonic speed = 340 m/s while Light speed 3x108 m/s
Thermal Radiation (열복사)
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What is "Temperature"?
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Temperature is a measure (or indicator) of the average kinetic energy
(운동에너지) of particles in a system.
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Thermal energy is the sum of all the individual kinetic energies of particles.
Thermal Radiation (열복사)
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Consider a solid-state object (e.g. star, metal, yourself)
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Photons of light within the object will bounce around from atom to atom exchanging
energy.
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The collisions among atoms randomize their kinetic energies and we can characterize
the average kinetic energy by the temperature of the object.
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The photons can escape the object once they reach its surface. Their energies now
only depend on the object's temperature. The radiation is called thermal radiation.
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The energy spectrum is a plot of the intensity of radiation vs. wavelength. It tells us
how many photons are being emitted at each wavelength.
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The shape of the curve depends only on temperature. There is a simple relationship
between the Temperature (in Kelvins) of thermal emitters and the peak of their energy
spectrum.
연속복사선 스펙트럼
흡수선 스펙트럼
Observer B
Observer A
Observer C
방출선 스펙트럼
생명서식가능지역
3. Time-scale and Distance
(시간 척도와 거리)
SI 단위계에서 사용하는 접두어
Cosmic Calendar
Cosmic Calendar: Scale 12 Billion year history of universe into one year.
1 month = 1 Billion years.
우주의 달력 : 우주의 탄생에서 현재까지를 1년으로 두었다
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Jan 1: The Big Bang
Feb. : The Milky Way forms
August 13: The Earth forms
December 13: Invertebrate (무척추) Life Evolves
December 25: Rise of dinosaurs
December 30: Dinosaurs extinct
December 31: 9:00 pm Earliest human ancestors
December 31: 11:59:47 pm Pyramids were built
December 31: 11:59:58.4 pm King Sejong was born
Chronology of the Universe
우주의 연표 (연대기)
13.7 Gyr
13.4 Gyr
12 Gyr
4.567 Gyr
4.5 Gyr
4.45 Gyr
4.4 Gyr
4.2 Gyr
4 Gyr
>3.5 Gyr
>3.5 Gyr
2.3 Gyr
0 Gyr
Big bang; formation of the elements H and He
First stars and galaxies; first supernova explosions produce the heavy
elements (C,N,O,Si,Fe,…)
Formation of the milky way
Formation of the solar system; at this point in time the interstellar
medium has been enriched with 1% heavy elements
Formation of the earth and the moon
Layer structure of the earth
Solid earth crust
Early ocean
Plate tectonics
Earth’s magnetic field
Origin of life
Formation of oxygen-rich atmosphere; formation of ozone
Today
Distance and Size of the Moon
달까지의 거리와 달의 크기
• In the 3rd Century B.C., a Greek, Aristarchus of Samos, figured out a
way to measure the relative sizes and distances of the Moon and Sun.
• He noticed that when the Moon was eclipsed by the Earth (월식) we
can see the Earth's shadow creep across the face of the Moon. Earth's
shadow is circular, and if we assume that the Sun's rays are roughly
parallel to one another.
• From the arc of Earth's shadow we can estimate the size of it's full
circle, then you measure the size of the Moon's circle. This tells us the
relative size of the Earth to the Moon. What you are measuring is the
angular size of the Earth's shadow and the Moon.
• Aristarchus measured a ratio of Earth's size to the Moon's size to be 2.9
(real ratio=3.7).
• Using the angular size of the Moon (=0.5 degree) we can estimate its
distance as shown below:
tan = D/d (when <<1,  in radian)
where D and d denote the diameter of the moon and distance between the
Earth and the moon.
• The angular size of the Moon is about 0.5° ( = 0.5°). Thus we have a
distance to the Moon of about 40 Earth Diameters (actually 30).
Distance and Size of the Sun
태양까지의 거리와 크기
• In addition, Aristarchus considered that when the Moon was seen to be
exactly in either 1st or 3rd Quarter Phase (exactly half lit up as seen
from Earth), we can draw a right triangle between Earth, Moon, and
Sun.
• The legs of the right triangle are the distance from the Earth to the
Moon (d1) and the distance between the Moon and the Sun (d2). The
hypotenuse is then the distance between the Earth and Sun (d). We can
measure the angle between d1 and d (A), it's the angular separation
between the Sun and Moon in the sky. We derive the distance d by the
Pythagorean theorem.
d2
d1
d
• Aristarchus measured A= 87°. This corresponds to a distance to the
Sun of d = 573DE. Now, the angular size of the Sun is also about 0.5°
so by the same method as for the Moon Aristarchus estimated that the
Sun was 5.33 times bigger than the Earth.
• This measurement is a difficult one to make, and the real answer is A=
89.9°. This corresponds to a distance to the Sun of d = 11,700DE and
a size of the Sun, DS = 109DE.
• Either way, Aristarchus was able to establish that the Sun was bigger
than Earth.
Aristarchus of Samos, the ancient Copernicus
• Earth radius
6400 km
• Moon-Earth distance
3.8105 km
• Sun-Earth distance 1.5108 km
1 AU (astronomical unit)
Parallax시차 (視差)
• Parallax = angle subtended (i.e., covered) by 1 A.U., the
distance from Earth to the Sun. (1/2 of the diameter of Earth's
orbit).
• As distance increases the parallax angle decreases. Ultimately
limited by the smallest angles measurable (resolved).
• The distance at which 1 A.U. subtends 1" (one arc second) is
called 1 parsec (pc). (parallax arc second).
• dpc = 1/p(")
• 1 pc = 3.26 Light-years. Distance to nearest star,  Centauri
p = 0.762" ---> d = 1.31 pc = 4.27 light-years
1 parsec = 3.26 light year