| Contents for this page | Related topics |  |
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Emission spectra Absorption spectra Lasers |
Transmission and scattering of light The photoelectric effect |
 Data Glossary |
| Learning Outcomes | ||
| After studying this section, you will (a) understand the basis for atomic emission and absorption spectra, and (b) understand the basic operating principle of lasers. | ||
| Helpful background knowledge | ||
| Electronic energy levels | ||
When an element is heated strongly, or when a gas at low pressure is subjected to a high electric potential, the element begins to glow, indicating that it is emitting energy in the visible region of the spectrum. This arises from electrons that have absorbed a certain amount of energy and have been promoted to a higher energy state. When the electrons revert to a lower energy state, they lose energy by emitting photons whose energy corresponds to the energy loss, E. From Planck's law, E = hc/l, where h is Planck's constant (6.63x10-34 J.s-1), c the velocity of light in a vacuum (3.00x108 m.s-1), and l the wavelength of the emitted photon.
In the case of hydrogen, the transitions that can occur are shown in the diagram below:
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Only the transitions down to n = 2 have energy values such that the associated wavelengths occur in the visible region of the spectrum. These give rise to the so-called Balmer series of lines (
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The more complex the electron configuration of the atom (remember that hydrogen has only one electron!), the more complex the spectrum becomes. Helium, with two electrons, has the emission spectrum shown below. The yellow line at 588 nm was detected in the spectrum of the sun in 1868, and was later recognised as being due to a hitherto unknown element, which was then called helium (from the Greek helios, meaning "sun"). Years later, it was found on earth in certain minerals.
Every element has its own distinctive emission spectrum. In a mixture of elements, each element will contribute its own set of distinctive lines, thus enabling one to establish which elements are present in the mixture. Astronomer can also in this way determine the elements that are present in very distant stars.
The spectrum of white light shows an uninterrupted change in colour from red to violet, as shown above. In 1814, Fraunhofer while repeating Newton's famous dispersion experiment using his own invention, the spectroscope, found that the spectrum of light from the sun showed a great many dark lines, which came to be known as "Fraunhofer lines". |
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This phenomenon occurs because vapour from various elements in the sun absorb light at certain wavelengths, coinciding with the wavelengths at which those elements emit light when their electrons are excited. Not all possible lines occur, since the absorption causing certain lines may be weak. On the left (top), we see a section of the visible absorption spectrum of sodium, compared with its emission spectrum (bottom). The pair of lines centered at 589.3 nm (the sodium D lines) is a "signature" for that element. |
A LASER (
) is a device that produces an intense coherent beam of nearly pure monochromatic light (). In a laser, atoms are energised in such a way that they emit light in phase, and not randomly, hence the coherence.
Thre are all sorts of different types of lasers. One of the simplest is the helium/neon laser, shown above. In this device, electrons of helium atoms are energised to the 2s state, and, during collisions with neon atoms, excite electrons of the neon atoms to the 5s state with an energy of 20.66 eV () above the ground state. These electrons drop to the 3p state, with energy of 18.70 eV. Applying Planck's law, the difference, 1.96 eV corresponds to radiation with wavelength 663 nm. The laser will therefore emit red light
This light is reflected backwards and forwards, some "leaking through" the mirror having 95% reflectivity. Most of the beam excites more atoms, so that the bulk of the neon atoms are in the 5s state. The 5% of the light that escapes passes through a very small hole, thereby creating the laser beam.
Lasers find very wide applications. They are used to read and write data to CD's and DVD's, in surveying, in gun sights, eye surgery, and as carving and drilling tools.
