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ASTR 151 Chapter 3

by: Wesley Fowler

ASTR 151 Chapter 3 ASTR 151 001

Marketplace > Astronomy > ASTR 151 001 > ASTR 151 Chapter 3
Wesley Fowler

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This is a bundle of notes covering the entirety of chapter three, which explores topics such as light, wave motion, the electromagnetic spectrum, electromagnetism, radiation, and the doppler effect...
Journey Thr Solar Sys Lecture
Dr. Sean Lindsay
astronomy, light, Electromagnetic spectrum, radiation, Wien, Stefan, doppler effect, wave
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This 6 page Bundle was uploaded by Wesley Fowler on Friday February 12, 2016. The Bundle belongs to ASTR 151 001 at a university taught by Dr. Sean Lindsay in Spring 2016. Since its upload, it has received 96 views.


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Date Created: 02/12/16
Wesley Fowler ASTR Chapter 3 Properties of Light Light is a form of radiation. It is the reason why we know anything about the universe. Light is also called electromagnetic radiation. Light is both a wave and a particle. Radiation is any way in which energy is transferred through space without physical connection. - Opacity: The extent as to how much radiation is blocked due to an object’s material. All light travels at the speed of light, no exceptions! (3.00x10 km/s) Wave Motion Frequency: Number of crests that pass a given point per second. Kind of like speed (hertz) - Frequency = (1/Period) Period: The time it takes for the crest of a wave to travel to another crest’s former position (s) Amplitude: The maximum departure or size of a wave from its undisturbed state Wavelength and frequency are inversely related. Big waves have small frequencies. Wavelength units: -9 Nanometer (nm) 1.0x10 m = 1nm Micrometer (m) 1.0x10 m = 1m Angstrom (Å) 10 m = 1 Å “The Single Slit Experiment” Thomas young - Proves that light can be a wave. The shadow from the experiment was fuzzy due to diffraction, it lost clarity. This shows that light can bend around corners, thus it is an electromagnetic wave. Charged Particles and Waves Electromagnetism is one of the four fundamental forces, along with gravity. Unlike gravity, electrical forces can either attract or repel objects. Light, as an electromagnetic wave, requires no medium to travel through. It is created by accelerating charged particles (protons and electrons attract, protons and protons repel) Any charged particle has an electrical field - Decreases with distance, same as gravity Any changing electrical field has a magnetic field - Moving charges create magnetic fields - Magnetic fields exert force on moving electrical charges Electrical and magnetic fields are two different aspects of Electromagnetism. They both comprise an electromagnetic wave. 5 - All electromagnetic waves travel at the speed of light (3.00x10 km/s) The Electromagnetic Spectrum: Spectrum: The division of light by specific wavelengths. In the visible spectrum, larger wavelengths are “reder”, shorter wavelengths are “bluer” - “Ultraviolet is bluer than visible light” Wavelength is typically measure in nanometers (nm) - Nano = 10 , 1 nanometer = 1x10 m (1 billion nanometers in a meter) -10 - Angstrom: (1 A = 10 m = 0.1 nm) Electromagnetic waves require no medium, unlike other types of waves. Created by accelerating charged particles. It can travel through a vacuum just fine. Atmospheric Opacity: How opaque the atmosphere is in allowing light through. 0% means unhindered passage, 100% means absolutely blocked. Wesley Fowler ASTR Chapter 3 Light and Radiation Thermal Radiation: Temperature and energy - Temperature is the measure of the average microscopic motion of particles, which collide, releasing energy, thus heat - Absolute zero: No motion, 0 Kelvin - Kelvin = Degrees Celsius + 273 (No degrees!) The average motion of particles Blackbody curve/spectrum: - Blackbody: An ideal object that absorbs and reradiates all forms of light perfectly (Coal is close, it absorbs 95% of light) The relationship between the intensity, or the amount of radiation (y axis), and frequency (x axis) - As temperature of blackbody increases, the peak of emission decreases to shorter wavelengths (“bluer”) Wien’s Law When things heat up to a certain temperature, they often produce a visible color. Wein’s Law: The peak of wavelength emission is inversely proportional to the temperature of the object, and is directly proportional to frequency. -  /T(K) peak - “Cooler stars have longer wavelength peaks” Wavelength: Lambda () Intensity refers to “the amount of color” Equations (Don’t need to be memorized): peakm(microns)) = (2900)/T(K) peaknm(nanometers)) = (2900)/T(1000s of K)) peakmm(millimeters)) = (0.29)/T(K) The color of a star reflects its surface temperature! Hotter stars are blue, cooler ones are red. Stefan’s Law: As temperature increases, energy output increases - The total energy emission per unit of time (Flux) is proportionate to 4 temperature Flux = T 4 Flux = Power/Area  = 5.67x10 W/m K (don’t need to memorize this) Power is energy per second. For Emitting objects this can be called Luminosity - For example, if you double the temperature of an area, the output of energy increases by a factor of (2)^4 = 16 **Both of these laws are used together in certain problems!** Example: If star 1 is 60K and star 2 is 600K… - Star 1: peakm(microns)) = 2900/60 = 48m - Star 2: peakm(microns)) = 2900/600 = - 4.8m 4 Star two’s flux increases by a factor of 10 Colors are both reflections of visible light, as well as emissions of light due to temperature. The Doppler Effect Relative motion influences how light and sound are perceived. Has to be towards or away from you (radial motion) - If an object is moving towards you, wavelengths seem shorter, and sound higher and appear “bluer”. - If an object is moving away from you, the wavelengths seem lower, and appear “redder” as they spread out over distance. This also applies if you are moving towards a stationary object. (Apparent wavelength/True wavelength) = (True frequency/Apparent frequency) = (Recession velocity/wave speed) - Depends on the relative motion between the source and observer, and it must have radial motion. It must move towards or away from the observer. It does not depend on distance!


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