Questions on Atomic Species. Atomic Models. Until the discovery of electrons and protons, both in the late 19th century, atoms were thought to be the most minute structure in nature. An atom was "modeled" by a point - the smallest imaginable point. But the discovery of atomic constituents required a more elaborate model than a point object model. It was initially proposed that atoms are made of positive charge centers scattered in a continuum of negative charge, similar to raisins in raisin bread.
This model seemed to agree rather well with experimental evidence, but further experimentation required major modifications. Raisin bread model of an atom with lumps of positive charge distributed throughout the negative charge. The most striking experimental evidence against such a model was that light emitted by well separated atoms like atoms in a gas or vapor was always missing some colors.
In fact, in the case of most atoms only a handful of colors were visible. These colors, known as the atomic spectrum, were produced in the laboratory by burning atomic vapors or by passing an electric current through them.
The consistency of the spectrum for a given atom clearly suggested that atoms emitted light of very distinct - signature - colors, unlike all other sources of light known. Further experimentation suggested that light itself appears to be made of separate, yet identical, constituents called photons. Furthermore, the energy of a photon is directly related to its color.
The more bluish the light the higher the frequency , the higher is the energy of the photons that make it up. This quantization of light, evident by its photon nature, along with atomic spectra then forced the notion of quantization of the atom. Quantization of the atom lead to Bohr's model that places the nucleus as the "sun" and electrons as planets revolving about it in fixed orbits analogous to our solar system.
The picture of this model is often drawn not as orbits in one flat plane, as is the case for the solar system, but in the atomic planetary model the orbits of electrons are three dimensional. This orbital model of the atom was in fact very effective at first, but it too was shown to be inadequate.
Despite the very limited use of the planetary model, it is this picture that is most widely used for depiction of the atom! Strangely enough, the most successful model, and the one that agrees well with currently accepted theories of quantum mechanics, is a hybrid of these two models. In the electron cloud model the "presence" of the electron about the nucleus resembles the continuum of negative charge, the bread, in the raisin bread model. But unlike the raisin bread model, and just as in the planetary model, here all the positive charge is concentrated in the center of the atom in one single raisin, i.
Questions on Atomic Models. The concept of quantization is not new to physics. In nature we are surrounded by quantized material of all sorts. Most living creatures but not all are quantized: they appear in separate units, as opposed to a continuum. Our neighbor may have one, two, three, Still, quantized properties are rare.
Most objects that we encounter in our every day lives have properties whose values can change continuously. But we find that in the case of small things, such as an atom, this is no longer the case. Most of the properties of very small things turn out to come in an integer multiple of a "smallest" unit called a quanta. In particular, light of a given color is made of identical photons. Each of these has an energy E g the value of which is a product of a constant, h, called Planck's constant , and the frequency of the photon, n.
There are other properties of very small sub microscopic entities that take just two values. One such property is the so called "spin".
Photons of any frequency have a spin value that is equal to Planck's constant divided by 2 p , no more and no less. Electrons and nucleons have half as much spin as do photons. This means that if two substances are to increase or decrease their net spin values by the exchange of electrons or nucleons, they can only do this in unit multiples of the spin of the electron or nucleon.
This, of course, is very similar to currency exchanges that take place only in multiples of the smallest unit of currency. Atomic quantization is a slightly different concept. Evidently, each atom is quantized based on that particular atom's makeup of electrons, protons, and neutrons and the interactions among these constituents.
The potential energy of the atom, because of the dynamic nature of its constituents, can have different values. But these values are very much limited to very specific ones. Said differently, in the continuum of energy values that each atom could possibly have, the energy values that the atom can actually have are limited to a set characteristic of its species.
A stone of mass m is attracted to the surface of the earth because of the gravitational force of interaction between it and the earth. Because this distance, H, can vary continuously, the energy can take any value. If we draw a vertical line depicting this height, and therefore this energy, then the stone can "be" at any location on this line. In this analogy, an atom can only be at very specific points on this line, similar to a ladder arrangement. Each atomic species, then, has a ladder whose rungs are separated according a characteristic scale that fits that species.
These rungs are referred to as the energy levels of the atom. An energy ladder of sorts; at least you need some to climb it! The lowest possible energy level of the atom is called its ground state.
In nature, evidently, all things tend to seek to their lowest possible energy situation. So, any atom tends to remain in its ground state, unless it is forced out of it. All the other rungs, then, are called excited states. The one just above the ground state is called the first excited state; the one above it, the second excited state; and so on.
In all atoms the rungs get closer and closer to each other the higher up this ladder we look. At the very top the rungs get so close to each other that for all practical purposes the energy scale gets to become a continuum. And objects at rest tended to remain at rest,. Light can also be produced by the acceleration of a free charged particle , such as an electron. The light emission is known as Bremsstrahlung or 'braking radiation'. The emission is characteristically seen in X-ray emission tubes which work by accelerating electrons with a high voltage and then by decelerating them very fast by directing them onto a metal target.
A special variety of particle accelerators known as Synchrotrons can be used to generate a wide range of light frequencies of very high power for use in the study of matter. A related effect is Cherenkov radiation which occurs when charged particles move through a medium faster than the speed of light. This produces the characteristic blue light seen in water ponds containing nuclear fuel. Discover more about Light Emission in our Learning Centre.
Part of the Oxford Instruments Group Expand. Oxford Instruments. In contrast, xenon arc lamps have a broader and more even intensity output across the visible spectrum, and do not exhibit the very high-spectral-intensity peaks that are characteristic of mercury lamps. Xenon lamps are deficient in the ultraviolet, however, and expend a large proportion of their intensity in the infrared, requiring care in control and elimination of excess heat when these lamps are employed.
The era of utilizing light emitting diodes as a practical source of illumination has arrived with the twenty-first century, and the diode is an ideal complement to the union of semiconductor technology and optical microscopy. The relatively low power consumption 1 to 3 volts at 10 to milliamperes , and long working life of light emitting diodes, renders these devices perfect light sources when low to medium intensity levels of white light are required.
Microscopes connected to computers interfaced through a universal serial bus USB port, or powered by batteries, can utilize the LED as a small, low-heat, low-power, and low-cost internal light source for visual observation and digital image capture. Several teaching and entry-level research microscopes currently utilize an internal, high-intensity white light emitting diode that serves as the primary light source.
Although the epoxy envelope light projection characteristics are still being explored, light emitting diodes are currently being tested and marketed in a wide variety of applications, such as traffic signals, signs, flashlights, and external ring-style illuminators for microscopy.
The light produced by white LEDs has a color temperature spectrum similar to that of a mercury vapor lamp, which is in the daylight illumination category. Examining the white LED emission spectrum presented in Figure 3, the transmission peak at nanometers is due to blue light emitted by the gallium nitride diode semiconductor, while the broad high-transmission range positioned between and nanometers results from secondary light emitted by a phosphor coating inside the polymer jacket.
The combination of wavelengths produces "white" light having a relatively high color temperature, which is a suitable wavelength range for imaging and observation in optical microscopy. Another source of visible light that is becoming increasingly more important in our everyday lives is laser illumination.
Among the unique features of lasers is that they emit a continuous beam of light composed of a single discrete wavelength or sometimes several wavelengths that exits the device in a single, aligned phase, commonly termed coherent light. The wavelength of light emitted by a laser depends upon the material from which the laser crystal, diode, or gas is composed. Lasers are produced in a variety of shapes and sizes, ranging from tiny diode lasers small enough to fit through the eye of a needle, to huge military and research-grade instruments that fill an entire building.
Lasers are used as light sources in a number of applications ranging from compact disk readers to measuring tools and surgical instruments. The familiar red light of the helium-neon often abbreviated He-Ne laser scans consumer purchases by lighting optical bar codes, but also plays a critical role in many laser scanning confocal microscopy systems. Despite the relatively high cost, lasers find particularly wide application in fluorescence, monochromatic brightfield, and in the rapidly growing fields of laser scanning confocal, total internal reflection, fluorescence resonance energy transfer, and multi-photon microscopy.
Explore how the argon-ion laser discharge tube operates with ionized gas to produce a continuous wave of light energy through the output mirror. The tutorial shows the slow build-up of light energy within the tube prior to establishing a steady state of laser discharge. Argon-ion lasers Figure 8 produce powerful spectral emissions at and nanometers, while krypton gas lasers exhibit large peaks at wavelengths of Both of these lasers are often utilized as excitation sources in laser scanning confocal microscopy.
Titanium-doped sapphire crystal mode-locked pulsed lasers are used as sources for multiphoton excitation due to their high peak intensity, but they also feature low average power and short duty cycles. As preferred light sources for multiphoton microscopy, pulsed lasers are considerably more expensive and difficult to operate than the small, air-cooled lasers employed in confocal microscopy. Newer laser technology features semiconductor-based laser diodes and single on-chip lasers that reduce the size and power requirements for light sources.
Laser diodes, such as neodymium:yttrium lithium fluoride Nd:YLF and neodymium:yttrium vanadate Nd:YVO 4 , typically are much faster in response than LEDs, but are also relatively small and require little power. Disadvantages of using lasers in microscopy include additional costs for the light source, the risk of expensive damage to optics, increased costs associated with lens and mirror coatings, destruction of specimens, and potential retinal damage to the microscopist if safe handling and operating techniques are ignored.
From this discussion, it is apparent that although there are a wide variety of available illumination sources, we generally rely on only a few throughout our everyday lives. During daylight hours the sun serves as our main source of illumination outdoors, while we generally rely on fluorescent and tungsten lighting while indoors and during the evening hours. As discussed above, these three primary lighting sources all have different properties and spectral characteristics, but their maximum intensities all fall within the visible light range.
The human brain adjusts automatically to the different light sources, and we interpret the colors of most objects around us as hardly changing when they are viewed under differing conditions of illumination. Michael W. Sources of Visible Light. Lightning: A Natural Capacitor Explore the build-up of static electrical charges between storm clouds and the wet ground during a thunderstorm with this tutorial, which simulates capacitor-like lightning discharges, one of nature's light sources.
Start Tutorial. Light Emitting Diodes Explore how two dissimilar doped semiconductors can be joined into a diode and produce light when a voltage is applied to the junction region between the materials. Color Temperature Discover how slowly heating a virtual black body radiator shifts the color spectrum of light emitted by the radiator from longer to shorter average wavelengths as the temperature is raised. Argon-Ion Gas Lasers Explore how the argon-ion laser discharge tube operates with ionized gas to produce a continuous wave of light energy through the output mirror.
Contributing Authors Kenneth R. Spring - Scientific Consultant, Lusby, Maryland, Electromagnetic Spectrum. Emission and Absorption. Field Spectroscopy. Photoelectric Effect. Field Spectroscopy First Draft. Greenhouse Gases. Lewis Dot Structures. Polarity, Dipoles, and Bonds. Molecular Shapes. Molecular Vibrations. Gapminder Statistics - BMI.
Oxidation-Reduction Reactions.
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