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------------ 2333 - Atoms - Rainbows Can Tell Us What the Universe is Made Of
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- In 1814 Joseph Von Fraunhofer did the prism experiment and found hundreds of fine dark lines amongst the colors. He eventually counted 600 lines. Today’s physicists have counted more than 30,000.
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----------------------- What are these dark lines and what do they mean?
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- In 1857 Robert Bunsen discovered that when he put certain chemicals into a hot gas Bunsen burner they emitted specific colors. (The Bunsen burner was named after him.) His colleague, Gustav Kirchhoff, suggested he pass the light from the colored flames through a prism. There were those dark lines again. More experiments showed that every chemical element produced its own unique pattern of dark lines.
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- In 1858, during a solar eclipse a new set of spectral lines was discovered that had not been seen before. They named the new element Helium after the Greek word Helios, meaning Sun. So helium was discovered on the Sun. It wasn’t discovered on Earth until 1895.
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- The atom is the smallest particle that has the properties of each particular element. In 1910 Ernst Rutherford, a physicists from New Zealand, proposed a new model for the structure of the atom. He concluded that the atom was composed of a positively charged nucleus surrounded be tiny negatively charged electrons. He saw the atom as mostly empty space with 99.95% of the mass concentrated in the nucleus inside a diameter 0.0001 the diameter of the atom.
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- Each element has a different number of protons in its nucleus. Hydrogen has 1, helium 2, oxygen 8, up to uranium that has 92 protons in its nucleus.
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- Normally the element’s atom is electrically neutral with the same number of electrons orbiting the nucleus as there are protons in the nucleus. If the number is different than the atom carries an electric charge, and it is called an ion.
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- If atoms share electrons in their outer orbits and remain bound together they are called molecules.
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- In 1911 Niels Bohr arrived at a deeper understanding of the structure of the atom. He assumed that electrons traveled in more than one orbit and that if they moved from one orbit to another they must gain or lose energy. With these ideas Bohr could explain the spectral lines of all the elements.
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- An absorption line is created when an electron jumps from an inner orbit to an outer orbit, absorbing the required photon form an outside source of energy. When an electron transitions from a higher orbits to a lower orbit the atom emits a photon of the required energy.
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- The values of the energy levels, E1, E2, etc , are different for each element atom. Therefore the absorption and emission spectral lines in the rainbow will occur at different frequencies that are unique to that element.
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- For example, for the four energy levels in the simplest hydrogen atom, which has only one electron and one proton, there are six different ways any electron can go between the four energy levels:
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----------------- Level four to level one
----------------- Level four to level two
----------------- Level four to level three
----------------- Level three to level one
----------------- Level three to level two
----------------- Level two to level one
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- Each of the four energy levels can be expressed in electron volts.
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----------------- Level one zero electron volts, the ground state
----------------- Level two 10.2 eV
----------------- Level three 12.1
----------------- Level four 13.6
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- Absorbing energy above 13.6 electron volts will cause the atom to lose its electron and become a positively charge ion, sometimes called an excited atom. Sometimes kids lose their electrons.
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- Each transition an electron makes between orbits causes it to absorb or emit a specific amount of energy. The exact frequency or wavelength of this energy can be calculated using Planck’s constant.
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----------------- Frequency = Change in energy level / Planck’s constant
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----------------- Planck’s constant = 4.136 * 10^-15 electron volt * seconds
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- For example when the hydrogen electron jumps from ground state, level one to level two:
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----------------- Frequency = 10.2 eV / 4.136 * 10^-15 = 2.466 * 10^15 cycles / second
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----------------- 2,466 terahertz is in the violet to ultraviolet frequency range.
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- The wavelength can be calculated by dividing the speed of light by the frequency:
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----------------- Wavelength = 299,792,458 m/sec / 2.466 * 10^15 cycles/sec
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----------------- Wavelength = 121.6 nanometers
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- Each of the energy levels changes in the hydrogen atom can be expressed in electron volts:
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----------------- Level four to level one = 13.6 eV
----------------- Level four to level two = 3.4
----------------- Level four to level three = 1.5
----------------- Level three to level one = 12.1
----------------- Level three to level two = 2.1
----------------- Level two to level one = 10.2
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- Each of these changes in electron energy level corresponds to a specific wavelength of emission or absorption, depending on which direction the electron is going.
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- Since both Planck’s constant and the speed of light are constants the formula can be rewritten to calculate the wavelengths directly:
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----------------- Wavelength = 12.4 * 10-7 / Change in energy level
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----------------- Level four to level one = 91.17 nanometers
----------------- Level four to level two = 364.7 nanometers
----------------- Level four to level three = 826.6 nanometers
----------------- Level three to level one = 102.4 nanometers
----------------- Level three to level two = 59.1 nanometers
----------------- Level two to level one = 121.6 nanometers
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- There you have the dark spectral lines seen in the rainbow. As you can see the spectral lines extend across the electromagnetic spectrum past the rainbow into ultraviolet, infrared, and still longer wavelengths.
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- Analyzing this spectrum of light, electromagnetic radiation, coming to us from the stars can tell us what each star is made of.
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- Not only that, once we understand the spectrum from a particular star we can analyze the gases and clouds of dust that are between us and that star. We observe the changes in the spectrum due to emission and absorption that occurs as the radiation passes through the gas and dust.
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- Not only that, once we understand the spectrum of a star and we observe that the spectral lines have shifted to longer or shorter wavelengths we know if the star is coming toward us or going away from us and how fast it is moving in that direction. This is called the redshift as most everything is moving away from us in our expanding universe.
It is also called the Doppler shift and uses the formula:
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------------------ Change in wavelength / At-rest wavelength = velocity / speed of light
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- For example, when we observe the spectrum of the star Vega we measure the hydrogen level four to one emission wavelength to be 656.255 nanometers. Normally, we would measure it in the lab to be 656.285 nanometers. The spectrum line is shifted 0.030 nanometers shorter.
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- Since it is a shift to shorter wavelengths it is called a blue shift, meaning that the star Vega is coming towards us.
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----------------- 0.030 nm / 656.285 nm = velocity of Vega / 3 * 10^8 m/s
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----------------- Velocity = 13,704 meters / second
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------------------ Vega is coming at us at 30,655 miles per hour.
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- See what can happen if you study the rainbow with a little more curiosity! It can tell us what the stars are made of and how fast they are traveling. Even new elements can be discovered through the telescope before they are found here on Earth. Think of that next time you see a helium balloon.
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- Here are some footnotes;
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- (1) Ernest Ruthford was born in New Zealand August 30, 1871. Rutherford’s grandfather was a Scotsman who migrated to New Zealand in 1842. His father was a wheelwright and a farmer with twelve children. Rutherford worked on the farm until he entered New Zealand University.
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- In 1895 he received a scholarship to attend Cambridge University. He was the first person in New Zealand to receive such a scholarship. While at Cambridge Rutherford discovered gamma rays.
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- He established the + sign for the positively charged nucleus of the hydrogen atom. In 1911 he announced the new concept for the structure of the atom and won the Noble Prize in 1908.
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- In 1933 he was in London strongly supporting the anti-Nazi campaign to help Jewish scientists escape from Germany. He died in 1937 at the age of 66 and was buried in Westminster Abbey next to Isaac Newton.
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- (2) Niels Henrik David Bohr was a Danish physicist born in 1885. He was the son of a physiology professor from the University of Copenhagen. Niels was an excellent soccer player. His younger brother was in the 1908 Danish Olympic soccer team that got a silver medal that year.
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- Niels went to Cambridge in 1912 and worked with Rutherford. Niels combined what he learned from Rutherford about the structure of the atom and the quantum theory of Max Planck to explain how substances emitted tha absorbed radiant energy.
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- In 1922 he won the Nobel Prize in physics. During the 1920 and 1930’s Carlberg brewery in Copenhagen sponsored the institute of atomic studies headed by Niels.
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- In 1943 Neils narrowly escaped German occupation by sneaking into Sweden. Form there he helped arrange the escape of nearly every Danish Jew for Hitler’s gas chambers.
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- In 1945 he helped the United States develop the atomic bomb as part of the Los Alamos project. After that he labored unremittingly on behalf of the peaceful use of atomic energy.
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- In 1957 he was awarded the Atoms of Peace prize. He died in 1962 at the age of 77.
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- April 14, 2019 38
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-------------------------- Sunday, April 14, 2019 --------------------------
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