AST101H5 Lecture Notes - Cosmic Ray, Mass Number, Gustav Kirchhoff

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Published on 23 Nov 2011
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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
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-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
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- 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, n0
!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 n0 has a mass number of 12 = carbon-12 = 12C
4. Number of neutrons is approximately equal to the number of protons
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