Atoms 127-134In order to understand what starlight is "trying to tell us" we will need to understand some very basic ideas of atomic structure and how light interacts with atoms.What is an Atom?Atoms are mostly empty space! Figure 6.1 shows a video clip of what we think a hydrogen atom might look like. Most of the atom's mass (99.95%) is contained in a tiny dot called the nucleus and situated at the center of the atom. The volume of the nucleus is only 1 part in 1015 (one thousand-trillionth!) the volume of the atom. Surrounding the nucleus - at very precise locations is a scintillating "cloud". This is an attempt to visualize one of the strange ideas of quantum mechanics - the electron around the atom does not orbit (like a mini-solar system) but has an assignable probability of existing - for a fleeting instant - at a particular position. This scintillating pattern is sometime referred to as an electron cloud.
Just How Does an Atom Produce Light?In order to understand how atoms produce light it is important to recall the idea of the photon introduced in the last chapter. One of the great discoveries of the early 20th century was made by Max Planck and Albert Einstein. Energy is exchanged by atoms in complete units called quanta (hence the name Quantum Theory) and the way in which quanta are exchanged is the photon! An atom gains or loses energy by either absorbing or emitting a photon.
Now Planck's radical idea comes on the scene. Normally, electrons try to stay in the lowest energy level open to them. BUT - if the right flavor of photon happens by they get EXCITED! The atom can absorb the photon because it has exactly the right energy to move the electron to one of its higher energy levels. An atom in which one or more electrons are in levels above the lowest possible ones is said to be in an excited state.
The Light - Energy ConnectionThere is a very convenient way to measure the energy of a photon of light.
As we already know, the shorter the wavelength or (equivalently) the higher
the frequency, the greater the energy of the light wave. Now that we are
using the photon idea we can express energy of a photon in the following
way:
or We can now give Planck's quantization idea a bit more detail. According to quantum physics energy is exchanged in strict accordance with the expression: so, if we are receiving energy from a light source of frequency "f" we can only receive energy in packages of size: hf, 2hf, 3hf .... and so on. (as a curious aside, the above formula combines both particle (photon) and wave (frequency) ideas! welcome to the strange world of quantum effects)How do we use the light - energy connection? We can now talk about atomic transitions and the photons either given off or absorbed in terms of energy exchanges. This will be very useful when we begin to discuss how light is actually being produced by an astronomical phenomenon.How to Excite an Atom
Continuum RadiationThe continuous "ROYGBIV" produced by an incandescent object provides an example of continuum radiation. Every wavelength is present from blue to deep red. (in principle - all wavelengths are present but the amount of light emitted drops rapidly as you move either direction away from the visible spectrum).Discrete RadiationThis is the type of light produced by individual atoms. Un -able to radiate at just any wavelength (equivalently: any color, equivalently: any energy) the atom produces a discrete set of colors. We will look at Hydrogen gas in just a moment.How to Make a SpectrumA spectrum will, in general be a combination of both discrete and continuous processes. A star surface for example, will have individual atoms producing a myriad of discrete "lines" while the electrons present in the gas will produce a background continuum.How Light Interacts With MatterNow the pieces are starting to come together. Imagine heating a gas to
a high temperature. Electrons will be liberated (ionization) and the atoms
will begin to jostle, get excited and radiate their characteristic colors.
Under this scenario we would be seeing a bright line spectrum. That
is, a spectrum dominated by bright emission lines from atoms present
in the gas. This is illustrated in the following: If, on the other hand we shine continuous light through a cool gas, the atoms in the gas will selectively filter out specific frequencies - in fact the same frequencies - and dark lines will appear. This is called a dark line spectrum. Our previous points about continuous, bright line (or emission) and dark line (or absorption) spectra are summarized in Kirchhoff's Laws:
A Case Study: Hydrogen and Its SpectrumHydrogen is the simplest atom and produces the simplest spectrum. By our good fortune it also makes up over 70% of the mass of the universe! So ... understanding the Hydrogen spectrum is very important.The following diagram is called an energy level diagram. We could have continued drawing concentric circles representing energy levels for an electron in a Hydrogen atom. More efficient, however, is this "flattened out" diagram that concentrates on the different energy levels and the transitions that can occur.
The Hydrogen Atom Applet
How Molecules Interact with Light to Help Create Spectral Windows We are now able to understand better how spectral windows are formed in the atmosphere. Molecules are far less "picky eaters" compared to simple atoms. This is because molecules have many more allowable energy levels than atoms and can absorb photons over very large ranges of wavelength in the electromagnetic spectrum.
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