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Lecture

Chapter 5

8 Pages
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Department
Astronomy
Course Code
AST101H5
Professor
John Lester

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Description
Astronomy Ch. 5 - Light: The Cosmic Messenger (Oct 7) Astronomical Evidence - Meteorites = solar system material that hits Earth - Moon rocks brought back by astronauts - Cosmic rays = very high energy subatomic particles from the Sun and deep space Meteorites + moon rocks + cosmic rays = almost nothing - Light = nearly all information, and only “connection” with distant objects Spectrum of Light - White light = visible light (seen by our eye) contains all colors - deep violet through deep red - White light has limited information -Sorting light by colors → spectrum of light = first step in getting info from light first done by Newton Properties of Light - Light is a wave - has a length, after which it repeats - has a height or amplitude above and below the central line - has a speed - has a frequency = number of waves passing a fixed location each second = speed/wavelength Light Waves - Wave speed = speed of light ▯ = 300,000,000 m/s = 3 x 10^8 m/s ▯ = 300,000 km/s = fastest possible speed - All wavelengths of light have the same speed in a vacuum - Symbol for the speed of light is “c” (constant) - Each color has its own wavelength - red light, wavelength = 0.0000007 m - violet light, wavelength = 0.0000004 m - Wavelengths of light are so small that using the meter, “m”, is inconvenient - Create new units - Micrometer = 0.000001 m = 10^ -6 m = μm ▯ Ex. red light = 0.7 μm ▯ - Nanometer = 0.000000001 m = 10^ -9 m = nm ▯ Ex. red light = 700 nm ▯ - Ångstrom = 10^ -10 m = Å ▯ Ex. red light = 7000 Å Frequency of Light - Frequency = speed of light / wavelength of light ▯ ▯ = highest possible speed / very tiny distance ▯ → frequencies are extremely large, too fast to measure ▯ wave/s is called Hertz = Hz Light Waves of What? - Light waves travel through space, but space is almost a perfect vacuum - Recall, light = radiation = energy - Energy in electric and magnetic fields - Wave = varying strength of electric and magnetic field = electromagnetic radiation - Only the electric component interacts with our eye or a camera - Brightness of light = (electric amplitude)^2 Many forms of “Light” - Our eye sees violet - red - Beyond red (longer wavelengths) we still receive energy = infrared radiation that we cannot see, but can feel - Below violet (shorter wavelengths) we still receive energy = ultraviolet radiation - Using different detectors, we find many other forms of electromagnetic radiation - Radiation often named after the detector Oct. 12 The Full Spectrum of Radiation - Radio - wavelength 1 m and longer ▯ frequency = 3 x 10^8 waves/s and less - Microwave - wavelength about 1 cm - Infrared - wavelength about 10 μm - Visible- wavelength about 500 nm - Ultraviolet - wavelength about 200 nm - X-rays - wavelength about 1 nm - Gamma (y) Rays - wavelengths less than about 0.01 nm ▯ frequency = 3 x 10^19 waves/s and greater Golden Age of Astronomy - Before 1950, all astronomy was done using visible light - Since then, astronomy has used all forms of radiation → tsunami of new info is coming in from using all forms of radiation → the most rapid advances in the history of astronomy = Golden Age Other Properties of Light - Light = energy wave - Light is also a particle = “photon” - Light is both a wave and a photon at the same time - Photon’s energy depends on the wave’s frequency - radio wave = low energy photons - gamma rays = high energy photons -Brightness of light ∝ number of photons/s What is Matter? - Light is important because it brings us info about distant matter - Fundamental form of matter = atom - About 100 different kinds of atoms = chemical elements - Hydrogen = most common element (NOT on earth) - Carbon, nitrogen, oxygen, iron, gold, etc... - On Earth, and in the Solar System, the temperature is cool enough that most atoms combine to form molecules - Nitrogen molecule - N:N = N2 - Oxygen molecule - O:O = O2 - Carbon Dioxide molecule - C:O:O = CO2 What is the Structure of Atoms? - To connect light to matter, we need to understand the structure of atoms - Three basic components: ▯ 1. Proton: positive electric charge, p+ ▯ 2. Neutron: no electric charge, n 0 ▯ 3. Electron: negative electric charge, e- Note: within the atom the electric force dominates and gravity is negligible How are Atoms Organized? - Mass of proton = mass of neutron - Protons and neutrons are confined to a tiny nucleus, about 10^-15 m across ▯ Note: nucleus has a + charge - Electron’s mass is negligible compared to the proton or neutron mass - Negative electrons fill out the volume of the atom, about 10^10 m across Oct. 14 Atomic Rules 1. Number of protons = “atonic number” → the chemical element ▯ 1 proton = hydrogen, 2 protons = helium ... 6 protons = carbon ... ... 2. Number of electrons = number of protons ▯ An atom has an exact balance of - and + electric charges ▯ → the whole atom has no electric charge 3. Number of protons + number of neutrons = “atomic mass number” ▯ ex. Carbin with 6 p + 6 n h0s a mass number of 12 = carbon-12 = 12C 4. Number of neutrons is approximately equal to the number of protons ▯ ex. Carbon with 6 p ++6 n = 0 12C ▯ - BUT different numbers of n0 are possible because they do not alter the charge ▯ balance: 6 p ++7 n = 0arbon-13 = 13C (also possible with 8) ▯ - 12C, 13, 14 are isotopes of carbon. On Earth: 98.9% 12C, 1.1% 13C How Do Light and Matter Interact? 1. Emission - matter with energy (“heat”) emits that energy as radiation 2. Absorption - matter takes in radiation, gaining its energy 3. Transmission - radiation passes through matter, with little absorption 4. Reflection or Scattering - radiation “bounces” off matter and goes in another direction Learning from Light - Recall - info is in the spectrum = light arranged by colour = wavelength - Many people studied the spectrum of light - Gustav Kirchhoff organized the results - “Kirchhoff’s Laws” Kirchhoff’s Laws of Radiation 1st law: A hot solid, liquid or dense gas emits radiation that varies smoothly with wavelength. - no gaps or breaks - a continuous spectrum ▯ Ex. the filament of an incandescent light bulb, or the heating element of a stove ▯ or a space heater - Type of heated material (aluminum, copper, iron, ...) does not affect the radiation, all produce the same continuous spectrum → you don’t learn anything about the substance - Spectrum does change with temperature → we can learn the temperature of the emitting material - Continuous radiation = “thermal” radiation Thermal Radiation - Two general trends: - The total amount of emission (summed over a
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