When a ray of light travels from one medium to another for example, from air through glass , the path can bend. The original ray is called the incident ray , which can be reflected bounces off a surface at same angle or refracted goes through substances at a different angle. Since most microscopes in the classroom use only light, we restrict our information to visible light rays.
Keep in mind that other wavelengths of the electromagnetic wave spectrum i. This high-energy form of radiation has wavelengths less than one-hundredth of a nanometer 10 picometers , photon energies greater than kiloelectron-volts keV , and frequencies exceeding 30 exahertz EHz. Exposure to gamma rays can induce mutations, chromosome aberrations, and even cell death, as is often observed in some forms of radiation poisoning.
However, by controlling the emission of gamma rays, radiologists can re-direct the high energy levels to combat disease and help cure some forms of cancer. Gamma ray astronomy is a relatively new science that collects these high-energy waves in order to produce images of the universe, as illustrated in Figure 5. This technique affords scientists opportunities to observe distant celestial phenomena in the search for new physical concepts, and to test theories that cannot be challenged by experiments performed here on the Earth.
X-rays - Electromagnetic radiation having a frequency just above the ultraviolet but below the gamma range is classified as X-rays, and is energetic enough to pass easily through many materials, including the soft tissues of animals.
The high penetration depths of these powerful waves, coupled with their ability to expose photographic emulsions, has led to the extensive use of X-rays in medicine to investigate textures in the human body, and in some cases, as a therapeutic or surgical tool. In the same manner as higher-energy gamma rays, uncontrolled exposure to X-rays can lead to mutations, chromosome aberrations, and other forms of cell damage. Traditional radiographic imaging methods essentially produce nothing more than shadow castings of dense material, rather than detailed images.
Recent advances in X-ray focusing technique using mirror optics, however, has led to significantly more detailed imagery from a variety of objects utilizing X-ray telescopes, X-ray microscopes, and interferometers.
Hot gases in outer space emit a powerful spectrum of X-rays, which are utilized by astronomers to gain information about the origin and characteristics of interstellar regions of the universe.
Many extremely hot celestial objects, including the sun, black holes, and pulsars, emit primarily in the X-ray region of the spectrum and are the subjects of astronomical X-ray investigations. The frequency spectrum of X-rays spans a very large range with the shortest wavelengths approaching the diameter of an atom. However, the entire X-ray spectral region traverses the length scale between approximately 10 nanometers and 10 picometers.
This wavelength range makes X-ray radiation an important tool to geologists and chemists for characterizing the structure and properties of crystalline materials, which have periodic structural features on a length scale comparable to the X-ray wavelengths.
Ultraviolet Light - Often abbreviated uv , ultraviolet radiation propagates at frequencies just above those of violet in the visible light spectrum. Although the low-energy end of this spectral region is adjacent to visible light, ultraviolet rays at the upper end of their frequency range have enough energy to kill living cells and produce significant tissue damage.
The sun is a constant source of ultraviolet radiation, but the atmosphere of the Earth primarily ozone molecules effectively blocks a majority of the shorter wavelengths of this potentially lethal radiation stream, thus affording a suitable living environment for plants and animals.
Photon energies in the ultraviolet are sufficient to ionize the atoms from a number of gas molecules in the atmosphere, and this is the process by which the ionosphere is created and sustained. Although small doses of this relatively high-energy light can promote the production of vitamin D in the body, and cause minimal tanning of the skin, too much ultraviolet radiation can lead to serious sunburn, permanent retinal damage, and the promotion of skin cancer.
Ultraviolet light is utilized extensively in scientific instruments to probe the properties of various chemical and biological systems, and it is also important in astronomical observations of the solar system, galaxy, and other parts of the universe.
Stars and other hot celestial objects are strong emitters of ultraviolet radiation. The ultraviolet wavelength spectrum ranges from about 10 to approximately nanometers, with photon energies ranging between 3. This category of radiation has applications in water and food treatment as an anti-microbial agent, as a photocatalyst for caged compounds, and is utilized to harden casts in medical treatments.
The germicidal activity of ultraviolet light occurs at wavelengths less than approximately nanometers. A market for blocking and filtering compounds employed in skin lotions, sunglasses, and window tints aims at controlling exposure to ultraviolet light from the sun.
Some insects notably honeybees and birds have sufficient visual sensitivity in the ultraviolet region to respond to longer wavelengths, and may rely upon this capability in navigation.
Humans are limited in their sensitivity to ultraviolet radiation due to absorption by the cornea of shorter wavelengths, and by strong absorption in the eye lens at wavelengths longer than nanometers. Visible light - The rainbow of colors associated with the visible light spectrum represents only about 2.
Color is not a property of the light itself, but the perception of color occurs through the combined response of the human eye-nerve-brain sensing system. The visible region of the electromagnetic spectrum lies within a narrow frequency band, from approximately to terahertz THz and is perceived as colors ranging from deep red wavelength of nanometers to deep violet nanometers.
The low-energy, long-wavelength red colors nanometers are followed in sequence by orange nanometers , yellow nanometers , green nanometers , blue nanometers , and finally, relatively high-energy, short-wavelength violet nanometers and below.
An easy method to remember the order in increasing frequency of the colors in the visible light spectrum is with the mnemonic acronym ROY G BIV R ed, O range, Y ellow, G reen, B lue, I ndigo, V iolet, , as taught to millions of school children for nearly a century although indigo is no longer considered a pertinent color by some scientists.
Division of the visible light spectrum into color regions based on physical properties is straightforward, but the manner in which color is sensed is not as obvious. Perception of color results from subjective responses of the human sensing system to the various frequency regions of the visible spectrum, and a variety of different combinations of light frequencies can produce the same visual response of "seeing" a specific color.
Humans may perceive the color green, for example, in response to a combination of light of several colors, none of which are necessarily composed of "green" wavelengths. Visible light is the basis for all life on Earth, and is captured by primary producers or autotrophs , such as green plants. These fundamental members of the biological food chain harness sunlight as the source of energy for manufacturing their own food and biochemical building blocks.
In return, autotrophs release oxygen, upon which all animals depend, as a by-product. In , Sir Isaac Newton studied the interaction of visible light with a glass prism and first recognized that white light is actually a mixture of different colors representing the entire visible light spectrum. White light originates from a variety of natural and artificial incandescent sources, including the sun, chemical reactions such as fire , and incandescent tungsten filaments.
The broad emission spectrum from sources of this type is referred to as thermal radiation. Other sources of visible light, such as gas discharge tubes, are capable of emitting light in narrow, well-defined frequency ranges representing a single color that depend upon specific energy level transitions in the source material atoms.
Strong perception of individual colors also results from specific absorption, reflection, or transmission characteristics of materials and objects that are illuminated with white light. The visible-ultraviolet light absorption spectrum of a common synthetic dye, Iris Blue B, is illustrated in Figure 6.
Solutions of this brilliantly colored organic molecule absorb light in both the visible and ultraviolet regions of the spectrum, and appear to most humans as a rich, medium blue color. Infrared Radiation - Often abbreviated IR , the large band of infrared wavelengths extends from the far-red portion of the visible light spectrum around nanometers to about one millimeter in wavelength. With photon energies ranging from approximately l. This type of radiation is associated with the thermal region where visible light is not necessarily detectable or even present.
For example, the human body does not emit visible light, but it does emit weak infrared radiation, which is felt and can be recorded as heat. The emission spectrum begins at about nanometers and ranges beyond the far infrared, peaking at approximately nanometers.
Molecules of all objects that exist above the temperature of absolute zero degrees Celsius emit infrared rays, and the amount of emission generally increases with temperature.
Approximately half of the sun's electromagnetic energy is emitted in the infrared region, and household items such as heaters and lamps also produce large quantities. Incandescent tungsten-filament lamps are rather inefficient producers of light, actually emitting more infrared than visible waves. Common tools that rely on detection of infrared radiation are night vision scopes, electronic detectors, sensors in satellites and airplanes, and astronomical instrumentation.
So-called heat-seeking missiles used by the military are guided by infrared detectors. In outer space, infrared wavelengths of radiation map the celestial dust between stars, as evidenced by the large dark patches visible from Earth when viewing the Milky Way Galaxy. In the household, infrared radiation plays a familiar role in heating and drying clothes, as well as allowing the remote control operation of garage doors and home entertainment components. Infrared photography takes advantage of the near-infrared spectrum to record images on specialized film useful in forensics, remote sensing such as aerial crop and forest surveys , painting restorations, satellite imaging, and military surveillance applications.
Curiously, infrared photographs of sunglasses and other optical surfaces coated with ultraviolet and visible light-blocking agents appear transparent, and reveal the eyes behind seemingly opaque lenses.
Infrared photographic film will not record thermal radiation heat distribution because it is not sufficiently sensitive to long-wavelength radiation far-infrared. Presented in Figure 7 are several infrared sensor-generated satellite images of two American cities and Mount Vesuvius, in Italy.
Microwaves - Currently the basis of a widespread technology utilized in millions of households for heating food, microwave spectral wavelengths range from approximately one millimeter to thirty centimeters or about one foot.
This chapter provides a practical basis for applying knowledge of electromagnetic energy to optical microscopy. The number of crests that pass a given point within one second is described as the frequency of the wave. One wave—or cycle—per second is called a Hertz Hz , after Heinrich Hertz who established the existence of radio waves.
A wave with two cycles that pass a point in one second as shown for the top wave in Figure 2. Electromagnetic waves have crests and troughs similar to but not identical in behavior to those of ocean waves. The distance between crests is the wavelength. The figure below shows the wavelength meters and energy Hz ranges for different types of electromagnetic energy:. The visible light region is divided into color regions Figure 2.
We will use colors and color changes due to different microscopy techniques to help identify minerals under thin section. Light waves across the electromagnetic spectrum behave in similar ways. When a light wave encounters an object, it is either transmitted, reflected, absorbed, refracted, polarized, diffracted, or scattered depending on the composition of the object and the wavelength of the light. Diffraction is the bending and spreading of waves around an obstacle. It is most pronounced when a light wave strikes an object with a size comparable to its own wavelength.
In the case of visible light, the separation of wavelengths through diffraction results in a rainbow. The figure below shows the grooves on a CD diffracting visible light and producing iridescent colors. Scattering occurs when light bounces off an object in a variety of directions. The amount of scattering that takes place depends on the wavelength of the light and the size and structure of the object.
The sky appears blue because of this scattering behavior. Light at shorter wavelengths—blue and violet—is scattered by nitrogen and oxygen as it passes through the atmosphere. Longer wavelengths of light—red and yellow—transmit through the atmosphere. This scattering of light at shorter wavelengths illuminates the skies with light from the blue and violet end of the visible spectrum.
Even though violet is scattered more than blue, the sky looks blue to us because our eyes are more sensitive to blue light. Refraction is when light waves change direction as they pass from one medium to another. Light travels slower in air than in a vacuum, and even slower in water. As light travels into a different medium, the change in speed bends the light. Different wavelengths of light are slowed at different rates, which causes them to bend at different angles.
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