| Contents for this page | Related topics |  |
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Emission spectra Absorption spectra Lasers Additional questions |
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/λ, 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 λ 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 () narrow 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. 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, in military equipment, and as carving and drilling tools.
The operation of any laser rests on two phenomena, namely, STIMULATED EMISSION and POPULATION INVERSION. These will be explained below by considering the emission of a laser beam as being the result of a stepwise process.
Consider a sample of some material whose atoms can be excited from the ground state with energy E0 to a higher energy state E1 (this takes place by energising an electron so that it is promoted to a higher energy orbital). If energy is supplied to the atoms, some will be excited from the ground state E0 to the state with energy E1, as shown above, (a) → (b). Under normal conditions, or with the application of heat, the vast majority of the atoms will be in the ground state. Excitation requires a high energy input, such as may be provided by an electric discharge (this is called PUMPING).
On returning to the ground state, these atoms will emit a photon of energy E1 - E0 = hf, where f is the frequency of the photon and h is Planck's constant (b) → (c). These photons are emitted in random directions. Normally, only a relatively small number of atoms are in the higher energy state E1.
If however the photon collides with an atom in the energised E1 state, that atom is stimulated to emit another photon with energy E1 - E0 = hf, without affecting the energy or direction of motion of the photon that collided with the atom. In this way, one photon has given rise to two photons (d) → (e). This is what is known as STIMULATED EMISSION. This process goes on and on until there is an excess of atoms in the E1 state. This is known as POPULATION INVERSION.
Step 1: Now let us take a tube whose ends are closed by mirrors. The mirror on the left is 100% reflective, while the one on the right is 95% reflective. Any light incident to this mirror will be mostly reflected, but 5% will pass through. Let it be filled with a material whose atoms can be excited and undergo population inversion as discussed above. A few atoms are initially energised (these are shown in red) by absorption of energy. |
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Step 2: After pumping for a while, population inversion has taken place. |
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Step 3: The excited atoms that revert to the ground state E0 emit photons in all directions, and many are absorbed by the surface of the tube. However, large numbers are perpendicularly incident to the mirrors, and are reflected back along the path from which they came, stimulating light emission from energised atoms along the way. There is a back and forth reflection, but each time that the photons hit the mirror at the right, 5% of them get through. This goes on until all atoms have reverted to the ground state. In practice this take place extremely fast, and one produces in this way an intense short burst of light of frequency f.
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.
