2. Introduction to Physics and Astronomy (1) 2-1 2. Introduction to Physics and Astronomy (1) 2-1. Light and Thermal Radiation

Nature of Light Radiant energy (방사에너지) Interacts with matter via either: Emission: Light is released Absorption: Light is captured Reflection (반사), Refraction (굴절), Diffraction (회절) Moves through a vacuum with a constant speed, c=3108 m/s Exhibits Wave (파도,물결) Nature Exhibits Particle (입자) Nature

complement Diffraction involves a change in direction of waves as they pass through an opening or around an obstacle in their path. Diffraction of sound waves is commonly observed; we notice sound diffracting around corners or through door openings, allowing us to hear others who are speaking to us from adjacent rooms.

complement

Huygens–Fresnel principle complement Huygens–Fresnel principle Huygens's principle applied to the passage of a plane wave through an aperture. The envelope function shows the wave spreading outwards from the aperture in a way that is independent of the wavelength. The Huygens'-Fresnel principle states that a light wave may be represented by an infinite number of hemispherical light sources incident on the wave front.

complement Diffraction involves a change in direction of waves as they pass through an opening or around an obstacle in their path. Diffraction of sound waves is commonly observed; we notice sound diffracting around corners or through door openings, allowing us to hear others who are speaking to us from adjacent rooms.

 

Monochromatic ==> 단색의 Visible light is the light that human eyes are sensitive to. Different colors correspond to electromagnetic waves having different wavelengths - Violet 400 nm - Blue 480 nm - Green 500 nm - Yellow 580 nm - Red 700 nm Wavelength or  White light is then made up of equal numbers of photons of all wavelengths. Monochromatic ==> 단색의

42 degree 42 degree

Discovery of Infrared (1800) William Herschel (1738-1822) Herschel passed sunlight through a glass prism to create a spectrum (the rainbow created when light is divided into its color components) and measured the temperatures of the different colors. He used three thermometers with blackened bulbs and placed one bulb in each color while the other two were placed outside the spectrum as controls. As he measured the temperature of the violet, blue, green, yellow, orange and red light, he noticed that all the colors had temperatures higher than the controls and that the temperature increased from the violet to the red part of the spectrum. After understanding this pattern, Herschel measured the temperature just beyond the red portion of the spectrum and found this area had the highest temperature of all and thus contained the most heat.

SI 단위계에서 사용하는 접두어

Light: Wave-Particle Duality (이원성) Before the 19th century, people knew about light was that it was bright, it was fast, and it came in a variety of colors. Very little was known about the nature of light. One of the great debates about light was over the question of whether light was made of a bunch of "light particles" or whether light was a wave.

Light as a wave Thomas Young (1773-1829) settled the question by performing an experiment in which he shone light through two narrow slits and observed the result. He placed a screen that had two slits cut into it in front of a monochromatic (or single color) light. Through the experiment, he proved the wave theory of the light. The wave theory predicted that light waves could interfere with each other like sound waves (beat) and sea wave. If light is a particle, then only the couple of rays of light that hit exactly where the slits are will be able to pass through. They will make a pattern of two exact lines on the viewing screen.

If light is a particle, If light is a wave, Suppose you have a bunch of ping-pong balls. You stand back about 10m from a doorway, and you dip the balls in paint and throw them through the door Well, you'll get a bunch of colored dots on the wall, scattered throughout an area the same shape as the door. This is how particles (such as ping-pong balls) behave. On the other hand, waves don't behave this way. Think of water waves. When a wave encounters an obstacle, it goes around it and closes in behind it. When a wave passes through an opening, it spreads out when it reaches the other side. And under the right conditions, a wave passing through an opening can form interesting patterns on the other side, which can be deduced mathematically.

Result Young found the interference (간섭) pattern with many stripes, indicating that light is a wave.

Light as Particles When light is shone on a metal surface, electrons can be ejected from that surface (photoelectric effect, 광전 효과). What Einstein showed is that if one assumes that light is made up of particles (photons, 광자), and if these particles have the properties described by Planck for his small bursts of light, then the photoelectric effect all makes sense. Einstein won the Nobel Prize based on this study.

(Energy) = hv = hc/ Independent of the intensity Only depends on the wavelength (Energy) = hv = hc/

Light can behave as discrete (이산의) particles: Photons Photons are little "wave packets" of energy Photons have energy but no mass. Quantum mechanics (양자역학) describes the energy, E, of a single photon with wavelength, as E = hc/ = hv where h is known as Planck's constant (상수, h = 6.626 x 10-34 Joules*seconds) This formula means shorter wavelength radiation has more energy than longer wavelength radiation.

Absorption and Emission

Why red?

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.

When an electron drops from a higher level to a lower level it must conserve energy. It must give up some energy, the amount being equal to the difference in the two energy levels.  Electromagnetic radiation The electron accelerates during the transition, which creates an electromagnetic wave as described earlier. The wave is a photon, whose energy is exactly equal to the difference in the energy levels, E = hc/. And so the wavelength (color) of the light emitted is dependent on how much energy it lost in the transition. Emission. An electron can jump up to a higher level if it can absorb a photon with just the exact energy (i.e. wavelength) equal to the difference in the two energy states. This process is called Absorption.

Absorption and Emission (흡수 와 방출) IR Visible Excited state UV 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

Sol-fa (Musical scale) Ut Re Mi Fa Sol La Si Ut Re Mi Fa Sol La Si Do Re Mi Fa Sol (So) La Si (Ti) 도 레 미 파 솔 라 시 262 Hz 294 Hz 330 Hz 349 Hz 392 Hz 440 Hz 494 Hz 523 Hz sa,re,ga,ma,pa,dha,ni French, Italy (English) Korean Sonic speed = 340 m/s

Doppler Effect (도플러 효과) the change in frequency (or wavelength) of a wave for an observer moving relative to the source of the wave.

can calculate the velocity of motion along the line of sight: 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

Temperature and Thermal Radiation

Heat Transfer(열(에너지) 전달) Convection is the transfer of heat energy in a gas or liquid by movement of currents. The heat moves with the fluid. Conduction is the transfer of energy through matter from particle to particle. It is the transfer and distribution of heat energy from atom to atom within a substance. Radiation: Electromagnetic waves that directly transport energy through space. Sunlight is a form of radiation that is radiated through space to our planet without the aid of fluids or solids. 대류 전도 복사

Vacuum bottle (진공병): A container that is used to keep liquids warm or cold for a long period of
time. It is designed to eliminate the heat transfer from one side to another side through conduction, convection and radiation.

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.

State of matter (물질의 상태 : solid, liquid, gas) A balloon full of gas has molecules moving inside, hitting repeatedly into the rubber walls and pressing them outward. In terms of energy… since there are molecules inside the balloon moving about, we must have kinetic energy inside the balloon because the molecules have both mass and velocity. Heat a balloon up and you give the molecules more energy: they absorb the heat you supply and move with more velocity, so they crash into the walls harder and exert more pressure. solid liquid gas Left: Solids are more dense than liquids: they have more atoms packed into the same space. The atoms are tightly packed together and stay in shape all by themselves, though they do move about on the spot. Middle: Liquids are less dense than solids but more dense than gases. Their atoms can move around much more, so they need a container to keep them in place. Right: Gases are even less dense than liquids.

What if you cool down a balloon at lower temperature? Suppose you fill your balloon with steam to start with. Cool it for a while and you'd get a balloon with a bit of water inside, then a balloon frozen with ice. If you keep on cooling, you take more and more energy from the molecules inside. The more you cool a solid, the less the atoms or molecules move. Could you reach a point where the atoms or molecules stopped moving altogether? Theoretically, yes. If you figure out the numbers, you'd need to cool down to -273.15°C (0 K, Kelvin), which scientists describe as the lowest temperature or absolute zero (절대온도 0도).

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 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. Spectrograph=분광기 Detector=검출기 Infrared=적외선

Black Body Radiation 흑체 복사 The energy radiated at different wavelengths by a black body at temperature T is given by Planck's law where h, k and c denote the Planck constant, Boltzmann constant and velocity of light respectively. The wavelength corresponding to the peak of the curve max can be represented by the equation T=7000 K T=6000 K T=5000 K known as Wein's displacement law. 빈의 변위법칙

April 2003 T=6000 K (solar type stars) T=300 K (habitable zone) T=30 K (molecular cloud) April 2003

Milky way and the Sun

Summary What can you learn from light? The Chemical Composition (화학조성) The Temperature (온도) The Velocity along the line of sight (시선방향의 속도)

연속복사선 스펙트럼 흡수선 스펙트럼 Observer B Observer A Observer C 방출선 스펙트럼

beyond …

생명서식가능지역 The Habitable Zone is a region where water can be liquid at normal atmospheric pressure.