Chapter in brief:
weak Balmer lines in stars cooler than sun - because of lack of atomic collisions,
Chemical composition: The chemical composition of most stars is found from the data of the spectra of the stars. Sodium D-lines - wavelength at 589 and 589.9 nm.
Model of an Atom: The planetary model of the atom consists of a tiny massive nucleus at the center and orbiting electrons surrounding the nucleus. Atom as a whole is electrically neutral. The nucleus contains protons that are positively charged, the electrons that are negatively charged and the neutrons that are electrically neutral particles. Size of an atom is measured in Angstorm (one of 10 billions parts of a meter) while that of the nucleus is 10,000 times smaller than Angstorm. Protons and neutrons are the particles that make up most of the mass of an atom.
Isotopes of an element have the same atomic number (equal to number of protons in the nucleus) but possess different mass because of deficiency or excess mass of neutrons.
A molecule is formed by two or more atoms bound together. Hot stars seldom permit atoms to form chemical bonds resulting in the formation of a molecule. Exceptions are the cool stars where Titanium oxide (TiO) is known to be present. Collisions among atoms in hot stars produce ions.
QUANTUM PHYSICS: When particles are small (e.g., atoms, electrons) and sizes are not large, the physical properties can be described by a set of rules different from everyday familiar notions. These rules or laws are what constitute the branch of physics called "Quantum Physics." One of the laws of quantum physics states that the location and speed of a particle can be simultaneously determined only with limited accuracy. The product of the uncertainties of location and its motion (momentum relating to speed) is always greater than a number called Planck's constant. This law challenges the REALITY of our ability to perform. What follows in these notes will use ideas of quantum nature and behavior of atoms and subatomic particles.
Electron shells: The ancient Greek scholars believed the motion of a planet to be generated by the rotating crystalline spherical shell. It was the model of the universe. In case of an atom, the picture is similar. The negatively charged electrons orbit around the center called the nucleus of an atom. The nucleus is massive containing positively charged protons and electrically neutral particles called neutrons. The electron and the positively charged nucleus are bound by an attractive electric force known as Coulomb's force. The energy needed to free the orbiting electron from the nucleus is called binding energy. The orbit radius of anyone electron is fixed in this model called Bohr model of an atom. This means that all orbits are not permitted for anyone electron. When an atom accepts a quantum of energy (called photon), the outer electron jumps from its stable orbit to a permitted higher orbit. This process is called absorption of a photon. When an atom releases a quantum of energy (called photon), the outer electron jumps from higher orbit to a permitted lower orbit. This process is called emission of a photon. The permitted orbits of the electrons in an atom depend on the charge of the nucleus or type of the atom.
Interaction of light and matter: Each permitted orbit in the atom has a specific amount of binding energy associated with it. This fixed energy of each orbit is referred to as energy level. When an atom moves from one energy level to another, the energy difference between the two levels, is the energy either absorbed photon or emitted photon. In the normal state of an atom, the outer electron stays in the lowest possible orbit. Such a state of an atom is called ground state. Whenever an atom absorbs a quantum energy the outer electron moves to higher orbit and, the atom is said to be in an excited state. This happens when the electron leaves its ground state. An atom may move to an excited state if it collides with another atom as in a hot gas. A photon may be absorbed by an atom, triggering an electron transition to a higher orbit or excited state.
Black body radiation: About one hundred years ago, the outcome of a simple experiment with heated object resulted in a break through or revolution in physics, called the quantum revolution. When a piece of iron is heated, it begins to glow with a color of deep dull red initially. When the heating is intense, the color changes to bright red, to orange, to yellowish white and finally to white color. This is because the electron in the iron atom makes transition to higher energy levels. Initially, the amount of energy absorbed is small resulting in deep dull red color (lower frequency). As the iron atom begins to absorb greater chunk of energy, it emits photon of higher energy such as yellow or green and blue photon. The final mixture of it seems like white color in our eye.
Radiation from a hot body. Wavelength of maximum intensity given by Wien's Displacement law.
Hydrogen spectrum: Characteristic spectral lines produced by heated hydrogen gas and viewed through a prism.
STELLAR SPECTRA: The 21cm radio frequency radiations from outer space suggest concentration of hydrogen atoms in the stars. The Balmer thermometer.
DOPPLER EFFECT: Everyone is familiar with the change in the intensity of siren from a moving Ambulance. This is called Doppler effect in sound. Similar effect is also detected in the case of light of certain wavelength.
Photons, Discrete Nature of Light:
Max Planck (1858-1947) argued emission of radiant energy by ideal radiators (blackbodies) is not continuous.
Energy transported by EM wave is not continuously distributed over wavefront defined by crests. Energy is actually located at discrete points, photons, along wavefront
Photons - discrete representation of electromagnetic radiation which carrys discrete or definite amounts of energy 1905, Einstein used Planck's idea of discrete nature for emission of light to explain a phenomenon discovered in 1887 known as photoelectric effect. Photon concept rests on extensive body of experimental and theoretical evidence. Conclusion today is that light does indeed exhibit a discrete nature
Properties of Photons: Energy content in photon inversely proportional to its wavelength. Shorter wavelength, more energetic is photon Longer wavelength, less energetic is photon. Equation for energy content:
- Energy of photon = hc/l
- where h = Planck'sconstant
- c = velocity of light
- l = wavelength
A hot body emits photons of discrete amounts of energy in all directions. Photons are created inside atoms of radiating body from which they receive their energy content. Photon energy content remains constant while traveling through space. Photons may be absorbed by atoms when they encounter matter; they lose their identity by transferring their energy to atom. Creation and destruction of photons by atoms is a classic example of conservation of energy.
Paradox of Light: Concept of light as being either particle or wave is quite possible. Light behaves as discrete photonsin some experiments and also, behaves as continuous waves in other experiments.
This concept seems self-contradictory and contrary to experience. When we think of discrete entities, marbles or pebbles, applicable concepts, such as size, precise location, etc, come to mind. For massless photons, such concepts have no meaning.
Max Born said, "The ultimate origin of the difficulty lies in the fact (or philosophical principle) that we are compelled to use the words of common language when we wish to describe a phenomenon, not by logical or mathematical analysis, but by a picture appealing to the imagination. Common language has grown by everyday experience and can never surpass these limits."
Laboratory experiments designed to inquire about either light's wave nature or its corpuscular nature. No experiment will simultaneously yield discrete and wave properties of light.
Max Born said, "We can therefore say that the wave and corpuscular descriptions are only to be regarded as complementary ways of viewing one and the same objective process, a process which only in definite limiting cases admits of complete pictorial interpretation...."
Mathematical resolution of paradox
Wavelength characterizes wavelike properties of light
Energy content refers to its discrete nature of light
Fact that wavelength and energy content can be linked in mathematical equation
(Energy of a photon = hc/l) strongest argument for duality ~ light cannot be simultaneously wave and photon.
The Balmer thermometer: The intensity in the Balmer series of visible spectral lines is related to the temperature of the gas body emitting the lines.
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