You are undoubtedly familiar with some of the other regions of the electromagnetic spectrum. The focus of Lesson 2 will be upon the visible light region - the very narrow band of wavelengths located to the right of the infrared region and to the left of the ultraviolet region. Though electromagnetic waves exist in a vast range of wavelengths, our eyes are sensitive to only a very narrow band. Since this narrow band of wavelengths is the means by which humans see, we refer to it as the visible light spectrum.
Normally when we use the term "light," we are referring to a type of electromagnetic wave that stimulates the retina of our eyes. In this sense, we are referring to visible light, a small spectrum from the enormous range of frequencies of electromagnetic radiation. This visible light region consists of a spectrum of wavelengths that range from approximately nanometers abbreviated nm to approximately nm.
Expressed in more familiar units, the range of wavelengths extends from 7 x 10 -7 meter to 4 x 10 -7 meter. Each individual wavelength within the spectrum of visible light wavelengths is representative of a particular color. That is, when light of that particular wavelength strikes the retina of our eye, we perceive that specific color sensation.
Isaac Newton showed that light shining through a prism will be separated into its different wavelengths and will thus show the various colors that visible light is comprised of. The separation of visible light into its different colors is known as dispersion.
Each color is characteristic of a distinct wavelength; and different wavelengths of light waves will bend varying amounts upon passage through a prism. The Sun is a natural source for visible light waves and our eyes see the reflection of this sunlight off the objects around us. The color of an object that we see is the color of light reflected. All other colors are absorbed. Light bulbs are another source of visible light waves.
Their design, construction, and operation are very simple, and a wide variety of these lamps have been utilized as incandescent light sources. Typical lamps consist of an sealed glass envelope see Figure 4 , evacuated or filled with an inert gas, and containing a tungsten wire filament that is energized by either direct or alternating current.
The bulbs produce a tremendous amount of light and heat, but the light accounts for only 5 to 10 percent of their total energy output. Tungsten lamps tend to suffer several drawbacks, such as a decreased intensity with age and a blackening of the inside envelope surface as evaporated tungsten is slowly deposited onto the glass.
The color temperature and luminance of tungsten lamps vary with the applied voltage, but average values for color temperature range from about K to K. The surface temperature of an active tungsten filament is very high, typically averaging 2, degrees Celsius for a standard watt commercial light bulb.
In some cases, tungsten bulb envelopes are filled with the Noble gases krypton or xenon inert fill gas as an alternative to creating a vacuum in order to protect the hot tungsten filament.
These gases improve the efficiency of incandescent lamps because they reduce the amount of evaporated tungsten that is deposited on the interior of the surrounding glass vessel. Halogen bulbs, a high-performance version of the incandescent tungsten lamp, typically contain traces of iodine or bromine in the fill gas, which return evaporated tungsten to the filament far more efficiently than lamps made with other gases.
Tungsten-halogen lamps, first developed by General Electric in the s for lighting the tips of supersonic jet wings, are capable of producing very uniform bright light throughout the bulb lifetime. In addition, halogen lamps are much smaller and more efficient than tungsten lamps of comparable intensity. The lifetime of a tungsten-halogen bulb can be as much as 10 years under the most ideal conditions.
The filaments of tungsten-halogen lamps are often very compact spiral assemblies mounted in a borosilicate-halide glass often termed fused quartz envelope. High operating temperatures restrict the use of tungsten-halogen bulbs to well-ventilated lamphouses with fan-shaped heat sinks to eliminate the tremendous amount of heat generated by these bulbs. Many household lamps are equipped to operate with watt tungsten-halogen lamps, and produce a significant amount light that fills a room much better than their weaker-emitting tungsten counterparts.
When coupled with fiber optic light pipes and absorption or dichromatic filters, tungsten-halogen lamphouses provide high intensity illumination for a wide variety of optical microscopy applications, but as a major disadvantage, produce significant amounts of infrared light in the form of radiant heat that can easily degrade the specimen.
There are a wide variety of non-incandescent visible light sources that are employed for indoor and outdoor lighting, in addition to having important applications in optical microscopy. Most of these light sources are based on electric discharge through a gas such as mercury, or the Noble gases neon, argon, and xenon.
The generation of visible light in gas discharge lamps relies on collisions between atoms and ions in the gas with an electrical current that is passed between a pair of electrodes placed at the ends of the bulb envelope.
The glass tube of a common fluorescent lamp is coated with phosphor on the inside surface of the glass, and the tube is filled with mercury vapor at very low pressure see Figure 5. An electric current is applied between the electrodes at the ends of the tube, producing a stream of electrons that flow from one electrode to the other.
When electrons from the stream collide with mercury atoms, they excite electrons within the atoms to a higher energy state. This energy is released in the form of ultraviolet radiation when electrons in the mercury atoms return to the ground state. The ultraviolet radiation subsequently energizes the internal phosphor coating, causing it to emit the bright white light that we observe from fluorescent lights. Fluorescent lamps are about two to four times more efficient at emitting visible light, produce less waste heat, and typically last ten to twenty times longer than incandescent lamps.
A unique feature of fluorescent light sources is that they generate a series of wavelengths that are often concentrated into narrow bands termed line spectra. As a consequence, these sources do not produce the continuous spectrum of illumination that is characteristic of incandescent sources. A good example of a almost exclusively single wavelength source of non-incandescent visible light is the sodium-vapor lamps commonly employed in street lighting.
These lamps emit a very intense yellow light, with over 95 percent of the emission being composed of nanometer light and virtually no other wavelengths present in the output.
It is possible to design gas-discharge lamps that will emit a nearly continuous spectrum in addition to the line spectra inherent in most of these lamps.
The most common technique is to coat the inside surface of the tube with phosphor particles, which will absorb radiation emitted by the glowing gas and convert it into a broad spectrum of visible light ranging from blue to red. Under normal circumstances, most individuals are not able to discern the difference between a line spectrum and a spectrum of continuous wavelengths.
However, some objects reflect unusual colors in light from a discontinuous source, particularly under fluorescent lighting.
This is why clothing, or other highly colored items, purchased in a store illuminated by fluorescent light often appears a slightly different color under natural sunlight or continuous tungsten illumination. 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.
In reflected light stereo microscopy, particularly when examining heat-sensitive specimens, fluorescent lamps are favored over tungsten lamps because of their high efficiency and low heat output.
Modern fluorescent lamps can be configured for linear tube or ring illuminators to provide the microscopist with intense, diffuse light. This source of artificial white light rivals sunlight without the accompanying heat in color temperature, and eliminates the flicker characteristics typical of consumer-grade fluorescent tubes.
In comparison to tungsten, tungsten-halogen, or arc lamps, fluorescent-lamp microscope illuminators can provide relatively long periods approximately 7, hours of high quality service. Wireless routers use light to send pictures of cats from the internet to your computer. Car radios use light to receive music from nearby radio stations. Even in nature, light carries many kinds of information. Telescopes are light collectors, and everything we know from Hubble is because of light.
Since we are not able to travel to a star or take samples from a faraway galaxy, we must depend on electromagnetic radiation — light — to carry information to us from distant objects in space.
The Hubble Space Telescope can view objects in more than just visible light, including ultraviolet, visible and infrared light. These observations enable astronomers to determine certain physical characteristics of objects, such as their temperature, composition and velocity.
The electromagnetic spectrum describes all of the kinds of light, including those the human eye cannot see. In fact, most of the light in the universe is invisible to our eyes. The light we can see, made up of the individual colors of the rainbow, represents only a very small portion of the electromagnetic spectrum. Other types of light include radio waves, microwaves, infrared radiation, ultraviolet rays, X-rays and gamma rays — all of which are imperceptible to human eyes.
All light, or electromagnetic radiation, travels through space at , miles , kilometers per second — the speed of light. Light travels in waves, much like the waves you find in the ocean.
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