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Today's contents • Review of Electromagnetic and Thermal Radiation • Time and Distance Nature of Light • • • 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=3108 m/s – Exhibits Wave (파도,물결) Nature – 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 • The electrons can only occupy discrete (이산의) energy levels within Atoms, Ions, and Molecules. • 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) • 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. • Amount of shift of wavelength is proportional to the component of velocity along the line of sight. • 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) • What is "Temperature"? • Temperature is a measure (or indicator) of the average kinetic energy (운동에너지) of particles in a system. • Thermal energy is the sum of all the individual kinetic energies of particles. Thermal Radiation (열복사) • Consider a solid-state object (e.g. star, metal, yourself) • Photons of light within the object will bounce around from atom to atom exchanging energy. • The collisions among atoms randomize their kinetic energies and we can characterize the average kinetic energy by the temperature of the object. • 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. • 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. • 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 • • • • • • • • • 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 3.8105 km • Sun-Earth distance 1.5108 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