Locate the centroid (x, y) of the shaded area. y 6 in. 3 in. 6 in. 6 in. x Prob. 962
Week 9 Notes Astr 100, Bob Berrington The Nature of Light Continued Constructive and deconstructive nature of light Longitudinal Waves o Compression wave Sound Alternating regions of high and low densities Displacement of air molecules in direction of travel Slinky example: Compress a part of the slinky that is stretched out. When you let go, the motion moves back and forth o Transverse Waves Sea waves Displacement perpendicular to motion of travel Light Waves Transverse waves of electric and magnetic fields Move the slinky up and down, that demonstrates a transverse waves Wavelength (λ) o Distance over which the wave repeats itself o Or, distance between 2 crests (highest point of each wave) Frequency (v) o Number of crests that passes a point in one second Period of wave = 1/v Frequency is the number of waves (cycles passing any point each second Relationship between frequency and wavelength o λv = (speed of wave) => for light this becomes λv = c Units o Visible light λ is often given in nanometer (nm) -9 o 1 nm = 1 x 10 meters Example o Calculate the frequency of yellow light Yellow light has λ = 500 nm = 5 x 10 meters V = c/λ, or v = 3 x 10 (m/z) / 5 x 10 m = 6 x 10 / s 14 6 x 10 / s cycles per second (also called a hertz) Notice units Units in cycles Spectrum o Intensity of Light versus Wavelength/Frequency Optical light goes from Violet (short λ, high v) to Red (long λ, low v) ROY G BIV (Red, Orange< Yellow, Green, Blue, Indigo, Violet) Electromagnetic Spectrum o Optical Spectrum is only a small part of the electromagnetic (EM) Spectrum o Radio at the low v/long λ end o Gamma (y) rays at the high v/short λ o Order from low v to high V: radio, microwaves, infrared, optical ultraviolet, X rays, y rays o Not all EM spectrum makes it through the atmosphere Light as a particle o Packets of energy Energy is related to frequency E= hv = hc/ λ h = Planc’s constant (in back of book) Higher frequency photons have greater energy X rays penetrate skin and bone Ultraviolet light causes sunburns Infrared is felt as heat Wednesday ASTR 100 Notes Packets of energy o Energy is related to frequency E= hv = hc/ λ o h = Planc’s constant (in back of book) o Higher frequency photons have greater energy X rays penetrate skin and bone Ultraviolet light causes sunburns Infrared is felt as heat Electromagnetic Spectrum o Red has the lowest energy (frequency); and violet has the highest frequency in the visible spectrum o Radio has the longest wavelength and the lowest energy while Gamma rays have the shortest wavelength and highest energy Angstrom o Written as: -10 o 1 angstrom = 10 m Expresses wavelength of optical ight 1 nm = 10 angstroms Typical size of an atom Optical light is 4000 – 7000 angstroms It’s an older unit that is losing popularity but still very much in use Atomic structure o Atoms are made of proton, electrons, and neutrons o Most of us probably learned the Bohr Model in high school One part of that model is incorrect The electrons do not circle the nucleus in orbits, but swarm around it in a “cloud” o Protons (positive charge) and neutrons (no charge) are found in the nucleus o Electrons (negative charge) swarm the nucleus in a cloud (they do orbit, just not in a predictable manner) Opposite charges attract Like charges repel o The nucleus is about 100,000 times smaller than the atom itself Atomic structure continued o It takes energy to remove an electron or increase its energy Provided by photons Energy = conserved o Quantum mechanics allows only specific energies for an electron Specific energies are emitted or absorbed by an electron bound to an atom Gives rise to atomic spectra o Ionization levels Each “level” of the electron cloud has different amounts of energy expressed in n=1, n=2, etc. When an atom is ionized, that means an electron has achieved a higher level of energy, or has gone down a level. When the level goes down= emission When the level goes up = absorption Nuclear Energy o Protons and neutrons are bound to each other in the nucleus o Heavy atoms give up energy when they break up into constituent parts (fission) The mass of the parts is less than the mass of the initial nucleus Called nuclear fission 2 Stays true to Einstein’s E = mc Atomic bombs are powered by nuclear fission o The sun is powered by nuclear energy (fusion) The energy is released as a photon when the light atom is formed by combining two lighter atoms (fusion) Mass of constituents is greater than the final nucleus 4 Hydrogen atoms = 1 Helium Energy Units o MKS system We won’t use this one very often Just know that a Joule is a measurement of energy 2 2 1 J = Nm=kgm /s o CGS system Ergs (measurement of energy) = dyres cm = g=cm /s 2 2 Atomic scales are smaller (we use electron volts abbreviated as ev) 1 ev = 1.602 x 10-12erg 1 J = 1 x 10 erg Atomic Spectra o The difference of possible energy levels the electron can orbit in If an electron is knocked out of an atom it is ionized Photons with energy greater than the ionization energy will be absorbed o (The equation works the same way as the universal law of gravity, just different symbols/letters to represent values) o Electron orbits and energies are unique to the element Each element given by the periodic table has unique energy levels Each spectrum is therefore unique Atomic Designations o Each element on the periodic table is represented by a 1 or 2 letter code The following superscript gives the atomic weight Atomic weight = number of protons and neutrons in the nucleus Atomic number Can precede the letter designation (usually written as a subscript) Number of protons and neutrons in the nucleus 56 Example F26or 56 on the other side of Fe (iron) Atomic spectra o Spectra of each element is unique Each element has a “fingerprint” We can figure out the composition of something by looking at its spectra Spectra of Orion nebula shows superposition of hydrogen, helium, oxygen, neon, etc.