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

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Nature of Light Radiant energy ( 방사에너지 ) Interacts with matter via either: 1.Emission: Light is released 2.Absorption: Light is captured 3.Reflection, Refraction, Diffraction Moves through a vacuum with a constant speed, c=3  10 8 m/s –Exhibits Wave Nature –Exhibits Particle Nature

Wavelength or Visible light is the light that human eyes are sensitive to. Different colors correspond to electromagnetic waves having different wavelengths - Violet400 nm - Blue480 nm - Green500 nm - Yellow580 nm - Red 700 nm White light is then made up of equal numbers of photons of all wavelengths.

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.

Thomas Young ( ) 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. Light as a wave

If light is a particle, If light is a wave,

Result Young found the interference 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 ). If one assumes that light is a wave, then there are certain features of the photoelectric effect that simply seem impossible. 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/

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 = 6.63 x Joules*seconds) This formula means shorter wavelength radiation has more energy than longer wavelength radiation.

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. Interaction between light and atoms

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. How does it do that? 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. The more the energy difference the shorter (bluer) the wavelength. This process is called 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.

Molecules may also absorb and emit light of specific energies that correspond to discrete states of rotation and vibration.

Doppler Effect Objects in motion compress the light waves in front of them making them appear more blue, the light waves behind are stretched out and appear more red. 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.

Heat Transfer 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.

Black Body Radiation 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 known as Wein's displacement law. T=7000 K T=6000 K T=5000 K

In order to obtain max, we differentiate B with respect to and set it equal to zero. If we define T=6000 K T=300 K T=30 K

If we assume «hc/kT, Rayleigh–Jeans law Wien's law If we assume »hc/kT, The figure shows that the total amount of the radiation shifts gradually from the ultraviolet to the visible to the infrared spectral regions as the temperature goes from a higher value to a lower value. The energy density u of the radiation field is related to B through the relation The total energy density is given by where  is the radiation constant. This equation is known as the Stephan-Boltzmann law. (3)

Milky way and the Sun

Summary What can you learn from light? Breaking light from an object into a spectrum can tell you: –The Chemical Composition –The Temperature –The Velocity along the line of sight