How Tattoo Removal Works

8/3/2019 How Tattoo Removal Works

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How Tattoo Removal Works

So you didn't believe your Mom when she said you'd regret getting that tattoo --the multicolored, fire-breathing dragon that starts at the small of your back,reaches up to your shoulder blades and wraps its orange flames around yourbiceps. Now, a mere seven years later, you have a shot at a terrific job inbanking, still one of the more conservative businesses around, and you areconcerned that your symbol of youthful self-expression could create problems inyour new career.

Well, you're not alone. Tattoos have become part of American mainstreamculture over the past couple of decades. Some estimate that more than 10 millionAmericans have at least one tattoo, and there are about 4,000 tattoo studios nowin business in the United States. One busy professional who specializes in tattooremoval -- he's removed tattoos from some of the most famous tattoo artists --estimates that about 50 percent of those who get tattoos later regret them. Foryears, these people had little recourse, and existing removal techniques wereinvasive (requiring surgery) and painful. But that's changing.

In this edition we'll examine how new laser tattoo removal techniques are helpingpeople of all ages rid themselves of something that, for a variety of reasons, theyno longer want on their bodies. (Falling out of love and wanting a no-longer-special person's name removed is the most popular reason cited, experts say!)

What Is a Tattoo?

Let's quickly remind ourselves exactly what a tattoo is: A tattoo is a permanentmark or design made on the body when pigment is inserted into the dermal layerof the skin through ruptures in the skin's top layer

Modern-day tattoos are applied by using an electric tattoo machine with needlesthat rapidly puncture the skin with an up and down motion not unlike that of asewing machine.


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Can All Tattoos Be Removed?

Most dermatologic surgeons caution that complete tattoo removal is notpossible. Tattoos are meant to be permanent, so removing them is difficult. Few

surgeons guarantee complete removal. Having said that, there are severalmethods of tattoo removal, which have proven effective. The degree of remainingcolor variations or blemishes depends upon several factors, including size,location, the individual's ability to heal, how the tattoo was applied and how longit has been in place. For example, a tattoo applied by a more experienced artistmay be easier to remove since the pigment was evenly injected in the same levelof the skin. New tattoos may also be more difficult to remove than old ones.Doctors say they can't predict the exact degree of removal because theygenerally don't know which of the 100 tattoo inks available today were used. (TheU.S. Food and Drug Administration currently lists tattoo pigments as "coloradditives," which are intended only for application to the top layer of the skin.)

Consult with a removal specialist -- be sure to take a list of questions along.

What Methods Are Used for Tattoo Removal?

Before lasers became popular for tattoo removal starting in the late 1980s,removal involved the use of one or more of these often-painful, often scar-inducing surgeries:


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Dermabrasion, where skin is "sanded" to remove the surface and middle layers;

Cryosurgery, where the area is frozen prior to its removal;

Excision, where the dermatologic surgeon removes the tattoo with a scalpel andcloses the wound with stitches (In some cases involving large tattoos, a skin graftfrom another part of the body may be necessary.).


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Although the procedures above are still used in certain cases today, lasers(Light Amplification by the Stimulated Emission of Radiation) have becomethe standard treatment for tattoo removal because they offer a bloodless, lowrisk, effective alternative with minimal side effects. Each procedure is done on anoutpatient basis in a single or series of visits. Patients may or may not require

topical or local anesthesia.

As early as the 1960s, lasers had been developed for industrial uses. Whenresearchers developed lasers that emitted wavelengths of light in short flashescalled pulses, medical use became viable. These lasers can effectively removetattoos with a low risk of scarring, according to the American Academy ofDermatology . The type of laser used to remove a tattoo depends on the tattoo'spigment colors. (Yellow and green are the hardest colors to remove; blue andblack are the easiest.)The three lasers developed specifically for use in tattooremoval use a technique known as Q-switching, which refers to the laser'sshort, high-energy pulses:

the Q-switched Ruby,

the Q-switched Alexandrite,

the Q-switched Nd: YAG,

The YAG is the newest system in this class of lasers and particularly advanced inthe removal of red, blue and black inks.

How Do Lasers Remove Tattoos?

Lasers work by producing short pulses of intense light that passes harmlesslythrough the top layers of the skin to be selectively absorbed by the tattoopigment. This laser energy causes the tattoo pigment to fragment into smallerparticles that are then removed by the body's immune system. Researchers havedetermined which wavelengths of light to use and how to deliver the laser'soutput to best remove tattoo ink. (If you're wondering if the laser might alsoremove normal skin pigment, don't worry. The laser selectively targets thepigment of the tattoo without damaging the surrounding skin.)


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Does Tattoo Removal Hurt and What Can I Expect?The unfortunate thing about tattoos is that both getting them and having themtaken off can be uncomfortable. The impact of the energy from the laser'spowerful pulse of light has been described as similar to getting hot specks ofbacon grease on your skin or being snapped by a thin rubber band. (Comparethese descriptions to those of how it feels to get a tattoo in How Tattoos Work.)Because black pigment absorbs all laser wavelengths, it's the easiest to remove.Other colors, such as green, selectively absorb laser light and can only betreated by selected lasers based on the pigment color.

In preparation for a laser procedure, doctors recommend that non-aspirinproducts, like Tylenol, be used for minor aches and pains prior to the procedure,because aspirin and nonsteroidal anti-inflammatory agents such as Ibuprofencan produce pronounced bruising after treatment.

Further pre-treatment steps might include the application of a prescriptionanesthetic cream two hours before the laser session. It is wiped off just beforelaser surgery begins. (Some patients say they don't need this. Others prefer tohave a local anesthetic injected into the tattoo prior to laser therapy. Pinpointbleeding is sometimes associated with the procedure.) Then pulses of light fromthe laser are directed onto the tattoo, breaking up the pigment. Over the next few

weeks, the body's scavenger cells remove pigment residues.

More than one treatment, which actually only takes minutes, is usually needed toremove an entire tattoo -- the number of sessions depends on the amount andtype of ink used and how deeply it was injected. Three-week intervals betweensessions are required to allow pigment residue to be absorbed by the body.Following treatment, the doctor will apply an antibacterial ointment and dressingto the area, which should be kept clean with continued application of ointment as


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directed by your doctor. A shower or bath the day after treatment is okay, but thetreatment area should not be scrubbed. Your skin might feel slightly sunburnedfor a couple of days and the treated area may remain red for a few weeks. Thesite might also form a scab, which should be handled gently. After healing, thesites will gradually and continually fade.

Side effects of laser procedures are generally few but may include hyper-pigmentation, or an abundance of color in the skin at the treatment site, andhypo pigmentation, where the treated area lacks normal skin color. Otherpossible side effects include infection of the site, lack of complete pigmentremoval and a 5 percent chance of permanent scarring.

How Much Does It Cost to Remove a Tattoo?

Something to think about BEFORE you get that tattoo is the fact that having atattoo removed is much more expensive than having one put on. Laser tattoo

removal can range from several hundred dollars up into the thousands of dollars,depending upon the size, type and location of the tattoo and the number of visitsrequired. More bad news is that medical insurance generally doesn't pay fortattoo removal, since it is considered aesthetic or cosmetic in nature. (Traumatictattoos, which result from accidents or injury, are a different matter.)

Because this is a medical procedure, make sure to see a dermatologic surgeonwho specializes in tattoo removal. Check with the American Society for LaserMedicine & Surgery or the American Society of Dermatologic Surgeons for areferral or ask your own doctor for the name of a specialist in your area. (Sometattoo parlors also provide tattoo removal services. Before you sign on, make

sure the person doing the removal is associated with a medical doctor whospecializes in laser surgery! Tattoo removal, like tattoo application, carries with itthe risk of infection and must be handled with extreme care.

How Can A Retired Gang Member Remove a Tattoo?

When a gang member gets smart and wants to get out of the gang and findemployment, he or she finds it nearly impossible to do so. Tattoos turn offemployers. "People just look at me and know I was a gang member," said a 19-

year-old from Ojai when he went for an unsuccessful job interview.

Tattoos are a stigma that follows them through life. They find it difficult to get ajob. They find it difficult to have a lasting relationship. They find it difficult tobecome a productive member of society.


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When people begin to regret their tattoos, many resort to drastic measures.These extreme measures to remove gang tattoos illustrate the steps our youthwill take to get out of gangs, hoping to lead productive lives.

Participants are initially screened for attitude and motivation. The individual must

WANT to change their life and be willing to donate their time to various agenciesand companies. In exchange for volunteer time, they can have LTR treatments ata discounted price.


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How Light Worksby Craig Freudenrich, Ph.D.

We see things every day, from the moment we get up in the morning until we go tosleep at night. We look at everything around us using light. We appreciate kids'crayon drawings, fine oil paintings, swirling computer graphics, gorgeous sunsets,a blue sky, shooting stars and rainbows. We rely on mirrors to make ourselvespresentable, and sparkling gemstones to show affection. But did you ever stop tothink that when we see any of these things, we are not directly connected to it? Weare, in fact, seeing light -- light that somehow left objects far or near and reachedour eyes. Light is all our eyes can really see.The other way that we encounter light is in devices that produce light --incandescent bulbs, fluorescent bulbs, lasers, lightning bugs, and the sun. Eachone uses a different technique to generate photons.

Ways of Thinking About Light

You have probably heard two different ways of talking about light:There is the "particle" theory, expressed in part by the word photon.There is the "wave" theory, expressed by the term light wave.From the time of the ancient Greeks, people have thought of light as a stream oftiny particles. After all, light travels in straight lines and bounces off a mirror muchlike a ball bouncing off a wall. No one had actually seen particles of light, but evennow it's easy to explain why that might be. The particles could be too small, ormoving too fast, to be seen, or perhaps our eyes see right through them.

The idea of the light wave came from Christian Huygens, who proposed in thelate 1600s that light acted like a wave instead of a stream of particles. In 1807,Thomas Young backed up Huygens' theory by showing that when light passesthrough a very narrow opening, it can spread out, and interfere with light passingthrough another opening. Young shined a light through a very narrow slit. What hesaw was a bright bar of light that corresponded to the slit. But that was not all hesaw. Young also perceived additional light, not as bright, in the areas around thebar. If light were a stream of particles, this additional light would not have beenthere. This experiment suggested that light spread out like a wave. In fact, a beamof light radiates outward at all times.

Albert Einstein advanced the theory of light further in 1905. Einstein consideredthe photoelectric effect, in which ultraviolet light hits a surface and causeselectrons to be emitted from the surface. Einstein's explanation for this was thatlight was made up of a stream of energy packets called photons.


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Modern physicists believe that light can behave as both a particle and a wave, butthey also recognize that either view is a simple explanation for something morecomplex. In this article, we will talk about light as waves, because this provides thebest explanation for most of the phenomena our eyes can see.

What is Light?

Why is it that a beam of light radiates outward, as Young proved? What is reallygoing on? To understand light waves, it helps to start by discussing a more familiarkind of wave -- the one we see in the water. One key point to keep in mind aboutthe water wave is that it is not made up of water: The wave is made up of energytraveling through the water. If a wave moves across a pool from left to right, thisdoes not mean that the water on the left side of the pool is moving to the right sideof the pool. The water has actually stayed about where it was. It is the wave thathas moved. When you move your hand through a filled bathtub, you make a wave,because you are putting your energy into the water. The energy travels through the

water in the form of the wave.

Light waves are a little more complicated, and they do not need a medium totravel through. They can travel through a vacuum. A light wave consists of energyin the form of electric and magnetic fields. The fields vibrate at right angles to thedirection of movement of the wave, and at right angles to each other. Because lighthas both electric and magnetic fields, it is also referred to as electromagneticradiation.

Light waves come in many sizes. The size of a wave is measured as itswavelength, which is the distance between any two corresponding points on

successive waves, usually peak-to-peak or trough-to-trough. The wavelengths ofthe light we can see range from 400 to 700 billionths of a meter. But the full rangeof wavelengths included in the definition of electromagnetic radiation extends fromone billionth of a meter, as in gamma rays, to centimeters and meters, as in radiowaves. Light is one small part of the spectrum.

Light waves also come in many frequencies. The frequency is the number ofwaves that pass a point in space during any time interval, usually one second. It ismeasured in units of cycles (waves) per second, or Hertz (Hz). The frequency ofvisible light is referred to as color, and ranges from 430 trillion Hz, seen as red,to 750 trillion Hz, seen as violet. Again, the full range of frequencies extends

beyond the visible spectrum, from less than one billion Hz, as in radio waves, togreater than 3 billion Hz, as in gamma rays.

As noted above, light waves are waves of energy. The amount of energy in a lightwave is proportionally related to its frequency: High frequency light has highenergy; low frequency light has low energy. Thus gamma rays have the mostenergy, and radio waves have the least. Of visible light, violet has the most energyand red the least.


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Light not only vibrates at different frequencies, it also travels at different speeds.Light waves move through a vacuum at their maximum speed, 300,000 kilometersper second or 186,000 miles per second, which makes light the fastestphenomenon in the universe. Light waves slow down when they travel insidesubstances, such as air, water, glass or a diamond. The way different substances

affect the speed at which light travels is key to understanding the bending of light,or refraction, which we will discuss later.

Figure 2

So light waves come in a continuous variety of sizes, frequencies and energies.We refer to this continuum as the electromagnetic spectrum. Visible lightoccupies only one-thousandth of a percent of the spectrum.

Producing a Photon

Any light that you see is made up of a collection of one or more photonspropagating through space as electromagnetic waves. In total darkness, our eyesare actually able to sense single photons, but generally what we see in our dailylives comes to us in the form of zillions of photons produced by light sources andreflected off objects. If you look around you right now, there is probably a lightsource in the room producing photons, and objects in the room that reflect thosephotons. Your eyes absorb some of the photons flowing through the room, andthat is how you see.

There are many different ways to produce photons, but all of them use the samemechanism inside an atom to do it. This mechanism involves the energizing of

electrons orbiting each atom's nucleus. How Nuclear Radiation Works describesprotons, neutrons and electrons in some detail. For example, hydrogen atomshave one electron orbiting the nucleus. Helium atoms have two electrons orbitingthe nucleus. Aluminum atoms have 13 electrons orbiting the nucleus. Each atomhas a preferred number of electrons orbiting its nucleus.


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Electrons circle the nucleus in fixed orbits -- a simplified way to think about it is toimagine how satellites orbit the Earth. There's a huge amount of theory aroundelectron orbitals, but to understand light there is just one key fact to understand: Anelectron has a natural orbit that it occupies, but if you energize an atom you canmove its electrons to higher orbitals. A photon of light is produced whenever an

electron in a higher-than-normal orbit falls back to its normal orbit. During the fallfrom high-energy to normal-energy, the electron emits a photon -- a packet ofenergy -- with very specific characteristics. The photon has a frequency, or color,that exactly matches the distance the electron falls.There are cases where you can see this phenomenon quite clearly. For example,in lots of factories and parking lots you see sodium vapor lights. You can tell asodium vapor light because it is very yellow when you look at it. A sodium vaporlight energizes sodium atoms to generate photons. A sodium atom has 11electrons, and because of the way they are stacked in orbitals one of thoseelectrons is most likely to accept and emit energy (this electron is called the 3selectron, and is explained on this page). The energy packets that this electron is

most likely to emit fall right around a wavelength of 590 nanometers. Thiswavelength corresponds to yellow light. If you run sodium light through a prism,you do not see a rainbow -- you see a pair of yellow lines.Probably the most common way to energize atoms is with heat, and this is thebasis of incandescence. If you heat up a horseshoe with a blowtorch, it willeventually get red hot, and if you heat it enough it gets white hot. Red is thelowest-energy visible light, so in a red-hot object the atoms are just getting enoughenergy to begin emitting light that we can see. Once you apply enough heat tocause white light, you are energizing so many different electrons in so manydifferent ways that all of the colors are being generated -- they all mix together tolook white, as explained in one of the sections below.Heat is the most common way we see light being generated -- a normal 75-wattincandescent bulb is generating light by using electricity to create heat. However,there are lots of other ways to generate light, some of which are listed below:Halogen lamps - Halogen lamps use electricity to generate heat, but benefit froma technique that lets the filament run hotter.Gas lanterns - A gas lantern uses a fuel like natural gas or kerosene as thesource of heat.Fluorescent lights - Fluorescent lights use electricity to directly energize atomsrather than requiring heat.Lasers - Lasers use energy to "pump" a lasing medium, and all of the energizedatoms are made to dump their energy at the exact same wavelength and phase.Glow-in-the-dark toys - In a glow-in-the-dark toy, the electrons are energized butfall back to lower-energy orbitals over a long period of time, so the toy can glow forhalf an hour.Indiglo watches - In Indiglo watches, voltage energizes phosphor atoms.Chemical light sticks - A chemical light stick and, for that matter, fireflies, use achemical reaction to energize atoms.The thing to note from this list is that anything that produces light does it byenergizing atoms in some way.


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Making Colors

Visible light is light that can be perceived by the human eye. When you look at the

visible light of the sun, it appears to be colorless, which we call white. Andalthough we can see this light, white is not considered to be part of the visiblespectrum. This is because white light is not the light of a single color, or frequency.Instead, it is made up of many color frequencies. When sunlight passes through aglass of water to land on a wall, we see a rainbow on the wall. This would nothappen unless white light was a mixture of all of the colors of the visible spectrum.Isaac Newton was the first person to demonstrate this. Newton passed sunlightthrough a glass prism to separate the colors into a rainbow spectrum. He thenpassed sunlight through a second glass prism and combined the two rainbows.The combination produced white light. This proved conclusively that white light is amixture of colors, or a mixture of light of different frequencies. The combination of

every color in the visible spectrum produces a light that is colorless, or white.

Colors by Addition - You can do a similar experiment with three flashlights andthree different colors of cellophane -- red, green and blue (commonly referred to asRGB). Cover one flashlight with one to two layers of red cellophane and fasten thecellophane with a rubber band (do not use too many layers or you will block thelight from the flashlight). Cover another flashlight with blue cellophane and a thirdflashlight with green cellophane. Go into a darkened room, turn the flashlights onand shine them against a wall so that the beams overlap. Where red and blue lightoverlap, you will see magenta. Where red and green light overlap, you will seeyellow. Where green and blue light overlap, you will see cyan. You will notice that

white light can be made by various combinations, such as yellow with blue,magenta with green, cyan with red, and by mixing all of the colors together.

By adding various combinations of red, green and blue light, you can make all thecolors of the visible spectrum. This is how computer monitors (RGB monitors)produce colors.


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When Light Hits an Object

When a light wave hits an object, what happens to it depends on the energy of thelight wave, the natural frequency at which electrons vibrate in the material and thestrength with which the atoms in the material hold on to their electrons. Based on

these three factors, four different things can happen when light hits an object:

S T A R

When treating an object some of the lasers waves will Scatter, Transmit, Absorband some will Reflect. Hence the acronym STAR.

The waves can be reflected or scattered off the object. The object can absorb thewaves. The waves can be refracted through the object. The waves can passthrough the object with no effect. And more than one of these possibilities can

happen at once. The following five illustrations show these possibilities, withreflection and scattering illustrated separately.

Scattering is merely reflection off a rough surface. Incoming light waves getreflected at all sorts of angles, because the surface is uneven. The surface ofpaper is a good example. You can see just how rough it is if you look at it under amicroscope. When light hits paper, the waves are reflected in all directions. This iswhat makes paper so incredibly useful -- you can read the words on a printed pageregardless of the angle at which your eyes view the surface.Another interesting rough surface is Earth's atmosphere. You probably don't thinkof the atmosphere as a surface, but it nonetheless is "rough" to incoming white

light. The atmosphere contains molecules of many different sizes, includingnitrogen, oxygen, water vapor and various pollutants. This assortment scatters thehigher energy light waves, the ones we see as blue light. This is why the sky looksblue.

Transmission - If the frequency or energy of the incoming light wave is muchhigher or much lower than the frequency needed to make the electrons in thematerial vibrate, then the electrons will not capture the energy of the light, and thewave will pass through the material unchanged. As a result, the material will betransparent to that frequency of light.


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Most materials are transparent to some frequencies, but not to others. Forexample, high frequency light, such as gamma rays and X-rays, will pass throughordinary glass, but lower frequency ultraviolet and infrared light will not.You can read more about what makes glass transparent on this page.

Absorption - In absorption, the frequency of the incoming light wave is at or nearthe vibration frequency of the electrons in the material. The electrons take in theenergy of the light wave and start to vibrate. What happens next depends uponhow tightly the atoms hold on to their electrons. Absorption occurs when theelectrons are held tightly, and they pass the vibrations along to the nuclei of the

atoms. This makes the atoms speed up, collide with other atoms in the material,and then give up as heat the energy they acquired from the vibrations.

The absorption of light makes an object dark or opaque to the frequency of theincoming wave. Wood is opaque to visible light. Some materials are opaque tosome frequencies of light, but transparent to others. Glass is opaque to ultravioletlight, but transparent to visible light.

Reflection: The atoms in some materials hold on to their electrons loosely. Inother words, the materials contain many free electrons that can jump readily fromone atom to another within the material. When the electrons in this type of materialabsorb energy from an incoming light wave, they do not pass that energy on toother atoms. The energized electrons merely vibrate and then send the energyback out of the object as a light wave with the same frequency as the incomingwave. The overall effect is that the light wave does not penetrate deeply into thematerial.

In most metals, electrons are held loosely, and are free to move around, so thesemetals reflect visible light and appear to be shiny. The electrons in glass havesome freedom, though not as much as in metals. To a lesser degree, glass reflectslight and appears to be shiny, as well.


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A reflected wave always comes off the surface of a material at an angle equal tothe angle at which the incoming wave hit the surface.

You can see for yourself that reflected light has the same frequency as theincoming wave. Just look at yourself in a mirror. The colors you see in the mirror'simage are the same as those you see when you look down at yourself. The colorsof your shirt and hair are the same as reflected in the mirror as they are on you. Ifthis were not true, we would have to rely entirely on other people to tell us what welook like!

Refraction - Refraction occurs when the energy of an incoming light wave

matches the natural vibration frequency of the electrons in a material. The lightwave penetrates deeply into the material, and causes small vibrations in theelectrons. The electrons pass these vibrations on to the atoms in the material, andthey send out light waves of the same frequency as the incoming wave. But this alltakes time. The part of the wave inside the material slows down, while the part ofthe wave outside the object maintains its original frequency. This has the effect ofbending the portion of the wave inside the object toward what is called the normalline, an imaginary straight line that runs perpendicular to the surface of the object.The deviation from the normal line of the light inside the object will be less than thedeviation of the light before it entered the object.

The amount of bending, or angle of refraction, of the light wave depends on howmuch the material slows down the light. Diamonds would not be so glittery if theydid not slow down incoming light much more than, say, water does. Diamondshave a higher index of refraction than water, which is to say that they slow downlight to a greater degree.

One interesting note about refraction is that light of different frequencies, orenergies, will bend at slightly different angles. Let's compare violet light and redlight when they enter a glass prism. Because violet light has more energy, it takeslonger to interact with the glass. As such, it is slowed down to a greater extent thana wave of red light, and will be bent to a greater degree. This accounts for theorder of the colors that we see in a rainbow. It is also what gives a diamond therainbow fringes that make it so pleasing to the eye.


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Rainbows in Soap Bubbles

Have you ever wondered why soap bubbles are rainbow colored, or why an oil spillon a wet road has rainbow colors in it? This is what happens when light wavespass through an object with two reflective surfaces. When two incoming light

waves of the same frequency strike a thin film of soap, as seen in Figure 5 below,parts of the light waves are reflected from the top surface, while other parts of thelight pass through the film and are reflected from the bottom surface. Because theparts of the waves that penetrate the film interact with the film longer, they getknocked out of sync with the parts of the waves reflected by the top surface.Physicists refer to this state as being out of phase. When the two sets of wavesstrike the photoreceptors in your eyes, they interfere with each other; interferenceoccurs when waves add together or subtract from each other and so form a newwave of a different frequency, or color.

Basically, when white light, which is a mixture of different colors, shines on a film

with two reflective surfaces, the various reflected waves interfere with each other toform rainbow fringes. The fringes change colors when you change the angle atwhich you look at the film, because you are changing the path by which the lightmust travel to reach your eye. If you decrease the angle at which you look at thefilm, you increase the amount of film the light must travel through for you to see it.This causes greater interference.

Figure 5

Everything we see is a product of, and is affected by, the nature of light. Light is aform of energy that travels in waves. Our eyes are attuned only to those wavefrequencies that we call visible light. Intricacies in the wave nature of light explainthe origin of color, how light travels, and what happens to light when it encounters

different kinds of materials.

ReferencesHewitt, Paul G., (1999) Conceptual Physics, Third Edition, Scott-Foresman-Addison-Wesley, Inc., Menlo Park, Calif.Serway, Raymond A, and Jerry S. Faughn, (1999) Holt Physics, Holt, Rinehart,and Winston, Austin, Texas


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How Lasers Workby Matthew Weschler

Lasers show up in an amazing range of products and technologies. You will findthem in everything from CD players to dental drills to high-speed metal cuttingmachines to measuring systems. They all use lasers. But what is a laser? Andwhat makes a laser beam different from the beam of a flashlight?

Photo courtesy NASAThe Optical Damage Threshold test station at NASA LangleyResearch Center has three lasers: a high-energy pulsed ND:Yag

laser, a Ti:sapphire laser and an alignment HeNe laser.

The Basics of an Atom

There are only about 100 different kinds of atoms in the entire universe.Everything we see is made up of these 100 atoms in an unlimited number ofcombinations. How these atoms are arranged and bonded together determineswhether the atoms make up a cup of water, a piece of metal, or the fizz thatcomes out of your soda can!

Atoms are constantly in motion. They continuously vibrate, move and rotate.Even the atoms that make up the chairs that we sit in are moving around. Solidsare actually in motion! Atoms can be in different states of excitation. In otherwords, they can have different energies. If we apply a lot of energy to an atom, itcan leave what is called the ground-state energy level and go to an excitedlevel. The level of excitation depends on the amount of energy that is applied tothe atom via heat, light, or electricity.


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Here is a classic interpretation of what the atom looks like:

An atom, in the simplest model,consists of a nucleus and orbiting electrons.

This simple atom consists of a nucleus (containing the protons and neutrons)and an electron cloud. It s helpful to think of the electrons in this cloud circling

the nucleus in many different orbits. Although more modern views of the atomdo not depict discrete orbits for the electrons, it can be useful to think of theseorbits as the different energy levels of the atom. In other words, if we apply someheat to an atom, we might expect that some of the electrons in the lower-energyorbitals would transition to higher-energy orbitals farther away from the nucleus.

Absorption of energy:An atom absorbs energy in the form of heat, light, orelectricity. Electrons may move from a lower-energy orbit to ahigher-energy orbit.

This is a highly simplified view of things, but it actually reflects the core idea ofhow atoms work in terms of lasers.

Once an electron moves to a higher-energy orbit, it eventually wants to return to

the ground state. When it does, it releases its energy as a photon -- a particle oflight. You see atoms releasing energy as photons all the time. For example,when the heating element in a toaster turns bright red, atoms, excited by heat,releasing red photons, cause the red color. When you see a picture on a TVscreen, what you are seeing is phosphor atoms, excited by high-speed electrons,emitting different colors of light. Anything that produces light -- fluorescent lights,gas lanterns, incandescent bulbs -- does it through the action of electronschanging orbits and releasing photons.


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The Laser/Atom Connection

A laseris a device that controls the way that energized atoms release photons."Laser" is an acronym for light amplification by stimulated emission of

radiation, which describes very succinctly how a laser works. Although there aremany types of lasers, all have certain essential features. In a laser, the lasingmedium is pumped to get the atoms into an excited state. Typically, veryintense flashes of light or electrical discharges pump the lasing medium andcreate a large collection of excited-state atoms (atoms with higher-energyelectrons). It is necessary to have a large collection of atoms in the excited statefor the laser to work efficiently. In general, the atoms are excited to a level that istwo or three levels above the ground state. This increases the degree ofpopulation inversion. The population inversion is the number of atoms in theexcited state versus the number in ground state.

Once the lasing medium is pumped, it contains a collection of atoms with someelectrons sitting in excited levels. The excited electrons have energies greaterthan the more relaxed electrons. Just as the electron absorbed some amount ofenergy to reach this excited level, it can also release this energy. As the figurebelow illustrates, the electron can simply relax, and in turn rid itself of someenergy. This emitted energy comes in the form of photons (light energy). Thephoton emitted has a very specific wavelength (color) that depends on the stateof the electron's energy when the photon is released. Two identical atoms withelectrons in identical states will release photons with identical wavelengths.

Laser light is very different from normal light. Laser light has the followingproperties:

The light released is monochromatic. It contains one specific wavelengthof light (one specific color). The wavelength of light is determined by the

amount of energy released when the electron drops to a lower orbit. The light released is coherent. It is organized -- each photon moves in

step with the others. This means that all of the photons have wave frontsthat launch in unison.

The light is very directional. A laser light has a very tight beam and isvery strong and concentrated. A flashlight, on the other hand, releaseslight in many directions, and the light is very weak and diffuse.


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To make these three properties occur takes something called stimulatedemission. This does not occur in your ordinary flashlight -- in a flashlight, all ofthe atoms release their photons randomly. In stimulated emission, photonemission is organized.

The photon that any atom releases has a certain wavelength that is dependenton the energy difference between the excited state and the ground state. If thisphoton (possessing a certain energy and phase) should encounter another atomthat has an electron in the same excited state, stimulated emission can occur.The first photon can stimulate or induce atomic emission such that thesubsequent emitted photon (from the second atom) vibrates with the samefrequency and direction as the incoming photon.

The other key to a laser is a pair of mirrors, one at each end of the lasingmedium. Photons, with a very specific wavelength and phase, reflect off the

mirrors to travel back and forth through the lasing medium. In the process, theystimulate other electrons to make the downward energy jump and can cause theemission of more photons of the same wavelength and phase. A cascade effectoccurs, and soon we have propagated many, many photons of the samewavelength and phase. The mirror at one end of the laser is "half-silvered,"meaning it reflects some light and lets some light through. The light that makes itthrough is the laser light.

You can see all of these components in the following figures, which illustrate howa simple ruby laserworks. The laser consists of a flash tube (like you wouldhave on a camera), a ruby rod and two mirrors (one half-silvered). The ruby rodis the lasing medium and the flash tube pumps it.

1. The laser in its non-lasing state

2. The flash tube fires and injects light into the ruby rod. Thelight excites atoms in the ruby.


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3. Some of these atoms emit photons.

4. Some of these photons run in a direction parallel to theruby's axis, so they bounce back and forth off the mirrors. Asthey pass through the crystal, they stimulate emission inother atoms.

5. Monochromatic, single-phase, columnated light leaves theruby through the half-silvered mirror -- laser light!

Types of Lasers

There are many different types of lasers. The laser medium can be a solid, gas,liquid or semiconductor.

Solid-state lasers have lasing material distributed in a solid matrix (suchas the ruby or neodymium: yttrium-aluminum garnet "Yag" lasers). Theneodymium-Yag laser emits infrared light at 1,064 nanometers (nm). Ananometer is 1x10-9 meters.

Gas lasers (helium and helium-neon, HeNe, are the most common gaslasers) have a primary output of visible red light. CO2 lasers emit energy inthe far-infrared, and are used for cutting hard materials.

Excimer lasers (the name is derived from the terms excitedand dimmers)

use reactive gases, such as chlorine and fluorine, mixed with inert gasessuch as argon, krypton or xenon. When electrically stimulated, a pseudomolecule (dimmer) is produced.

Dye lasers use complex organic dyes, such as rhodamine 6G, in liquidsolution or suspension as lasing media. They are tunable over a broadrange of wavelengths.

Semiconductor lasers, sometimes called diode lasers, are not solid-state


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lasers. These electronic devices are generally very small and use lowpower. They may be built into larger arrays, such as the writing source insome laser printers or CD players.

A ruby laser(depicted on the previous page) is a solid-state laser and emits at awavelength of 694 nm. Other lasing mediums can be selected based on thedesired emission wavelength (see table below), power needed, and pulseduration. Some lasers are very powerful, such as the CO2 laser, which can cutthrough steel. The reason that the CO2 laser is so dangerous is because it emitslaser light in the infrared and microwave region of the spectrum. Infraredradiation is heat, and this laser basically melts through whatever it is focusedupon.

Other lasers, such as diode lasers, are very weak and are used in today s pocketlaser pointers. These lasers typically emit a red beam of light that has a

wavelength between 630 nm and 680 nm. Lasers are utilized in industry andresearch to do many things, including using intense laser light to excite othermolecules to observe what happens to them.

Here are some typical lasers and their emission wavelengths:

Laser Type Wavelength (nm)

Argon fluoride (UV) 193

Krypton fluoride (UV) 248

Nitrogen (UV) 337

Argon (blue) 488

Argon (green) 514

Helium neon (green) 543

Helium neon (red) 633

Rhodamine 6G dye (tunable) 570-650

Ruby (CrAlO3) (red) 694

Nd:Yag (NIR) 1064

Carbon dioxide (FIR) 10600

Laser Classifications

Lasers are classified into four broad areas depending on the potential for causingbiological damage. When you see a laser, it should be labeled with one of thesefour class designations:

Class I - These lasers cannot emit laser radiation at known hazard levels.


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Class I.A. - This is a special designation that applies only to lasers thatare "not intended for viewing," such as a supermarket laser scanner. Theupper power limit of Class I.A. is 4.0 mW.

Class II - These are low-power visible lasers that emit above Class Ilevels but at a radiant power not above 1 mW. The concept is that the

human aversion reaction to bright light will protect a person. Class IIIA - These are intermediate-power lasers (cw: 1-5 mW), which are

hazardous only for intrabeam viewing. Most pen-like pointing lasers are inthis class.

Class IIIB - These are moderate-power lasers. Class IV - These are high-power lasers (cw: 500 mW, pulsed: 10 J/cm2 or

the diffuse reflection limit), which are hazardous to view under anycondition (directly or diffusely scattered), and are a potential fire hazardand a skin hazard. Significant controls are required of Class IV laserfacilities.

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