Monochromatic Laser Light

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Monochromatic LASER Light Big Idea BIG IDEAS MONOCHROMATICDemonstrate LASER the monochromatic LIGHT Working with LASERs, it’s immediately obvious that they don’t nature of LASER light by Working with LASERs, it’s immediately obvious that they don’t • Demonstrate the behave like the light we encounter everybehave day. Whatlike the makes light LASERwe encounter everycomparing day. Whatthe spectra makes of LASER white monochromatic nature of light different from the light emitted bylight a normal different light from bulb ?the light emittedlight by anda normal colored light LEDs bulb? to that of LASER light by comparing LASER light. the spectra of white light WHAT IS COLOR? andWhat colored is color? LEDs to that Color is a property defined by the wavelength of light. Different ofColor LASER is a propertylight. defined by the wavelength of light. Different What You’ll Need colors of light have different wavelengths. When you look at a red colors of light have different wavelengths.object, When it looks you look red atbecause a red the object1 Diffraction is reflecting Grating red light. object, it looks red because the object is reflecting red light. 3 Lazr fingers/Light BLOX: Red, When light consists of only one wavelength of light, we say it is WHAT YOU’LL NEED Green, Blue monochromatic (from ancient Greek, mono meaning “alone” or • 1When Diffraction light consists Grating of only one wavelength“single”, of light, and wechromatic say it is meaning color).1 White Lazr finger • Lightmonochromatic BLOX: Red, (from Green, ancient Greek, mono meaning “alone” or 1 Flashlight (optional) WHITE LIGHT Blue“single”, and chromatic meaning color). 1 Red Laser Pointer • 1 White LED So what color is the light emitted fromA blank the wall/screen sun, or the overhead lights? Yellow? White? Does it even have a color? • 1White Flashlight Light (optional) • 1So Red what Laser color Pointer is the light emitted fromYes, the sun,it does! or the This overhead light is calledlights? whiteYellow? light, White? and Does despite it even its name, have a it’s actually made up of a combination of colors. White light from • Acolor? blank wall/screen the sun is made up of all the colors of you rainbow, it just looks white to you when they are all together. This is why you can see all Yes, it does! This light is called white lightthe ,different and despite colors its name, of objects it’s actually when themade sun up shines of a combination or from the of light RELATEDcolors. White PRODUCTS light from the sun is madeof a up flashlight. of all the colorsEvery ofwavelength you rainbow, of lightit just is looks available white toto yoube reflected when Click the below to be taken right or absorbed, so every color has the potential to be visible. theyto are the productall together. page. This is why you can see all the different colors of objects when the sun shines or from the light DIFFRACTION of a flashlight. Every GRATINGS wavelength of light Red Laser Blox is available to be reflected or absorbed,A diffractionso every color grating has the is potentiala transparent to be visible. piece of plastic mounted in a frame,

Light Blox as shown in Figure 1. What you can’t see, is that the plastic is covered Diffraction Gratings in thousands of tiny scratches. The A diffractionRed & Green grating Laser is a transparent piecescratches of plastic run mountedfrom the intop a frame,of the as Pointer shown in Figure 1. What you can’t see,plastic is that to the the plastic bottom, is covered and are in parallel Figure 1: Diffraction grating of the thousands of tiny scratches. The scratchesto one run another from the (in topthe ofsame the plasticdirection, to same kind as you have in your kit. the bottom, and are parallel to one another (in the same direction, but not touching). The lines are so thin that you can fit hundreds of scratches on one mm of plastic!

WWW.LASERCLASSROOM.COM When you shine a beam of light through a diffraction grating, it splits the light but not touching). The lines are so thin that you can fit hundreds of up into its component wavelengths. All of the different colors of light that make scratches on one mm of plastic! up the beam of light can be seen side by side, as in Figure 2 - this is called its spectrumWhen you. But shine what a beamhappens of lightif the through light isn’t a white,diffraction but monochromatic? grating, it splits the light up into its component wavelengths. All of the different colors of light that make up the beam of light can be seen side by side, as in Figure 2 - this is called its spectrum. But what happens if the light isn’t white, but monochromatic? Figure 2: Spectrum of white light

Monochromatic Light in Real Life: Spectrometry MONOCHROMATIC LIGHT IN REAL LIFE: SPECTROMETRY GasesGases absorb absorb light light that thatpasses passes through through them .them. But they But only they absorb only absorb specific specific wavelengths wavelengths,, and every gasand absorbsevery a gas different absorbs combina a differenttion of wavelengthscombination. Ifof scientists wavelengths. know Ifthat scientists a specific know gas, that and onlya specific that gas, absorbsgas, anda certain only thatwavelength gas, absorbs of light, a they certain can wavelengthtest for that gasof light, using they LASERs. can testNote: for again, that gasthis usingcould be LASERs. Note: again, this could be written “lasers” written “lasers” Scientists can pick a laser that is a certain wavelength, or use a laser that has a tuneable wavelength and shine it through the place where they suspect the gas exists. If reflected Scientists can pick a laser that is a certain wavelength, or use a laser that has a tuneable wavelength light from the LASER returns weaker in certain areas, it means that the gas must be present, andsince shine it itabsorbed through the some place of wherethe LASER they suspect light. This the techniquegas exists. isIf reflectedused in examining light from the the LASER Earth’s returns weakeratmosphere, in certain and areas the, it atmospheres means that the of gasother mus planets.t be present, since it absorbed some of the LASER light. This technique is used in examining the Earth’s atmosphere, and the atmospheres of other planets.

WWW.LASERCLASSROOM.COM ACTIVITY SHEET: MONOCHROMATIC LASER LIGHT

Let’s investigate one of the aspects of LASER light that makes it different from “normal” light.

• Hold the diffraction grating up to one eye.

• Hold the white Light Blox at arm’s length, pointing towards your eye, and turn the Light Blox on. Don’t look directly at the light; rather slightly to the left or right of the source.

What do you see? Record the order of the colors, starting from closest to the light source.

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White light is made up of a rainbow, all the colors mixed together. The order of colours are violet, blue, green, yellow, orange and red.

• Now hold the diffraction grating up to your eye and look at a flashlight, the overhead lights, or the sun.

Was the rainbow the same for all sources of white light? What do you expect to see for the colored Light Blox?

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This spectrum is the same for all white light; always in the same order.

• Repeat the process with the red, blue and green Lazr fingers.

What do the spectra (plural of spectrum) look like? Was this what you expected? Why/why not? What is colored light really made of?

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You might’ve expected to see only red light for the red Light Blox, blue for the blue Light Blox, etc. Actually, colored lights are not one colour at all. They consist of a spectrum of colors as well, they just have mostly the color that they look like. For example, when you look at the red Light Blox with the diffraction grating, it looks mostly red, but there is a little bit of yellow and green too. This means that the Light Blox are not monochromatic!

• Point the red LASER pointer towards a blank wall or screen, approximately 15-20 cm away from the surface. IMPORTANT: NEVER POINT A LASER AT YOUR FACE! Never look into a laser or point one at someone else’s face either.

WWW.LASERCLASSROOM.COM • Hold the diffraction grating about 5-10 cm from the front of the LASER, and switch on the LASER pointer.

How is this different to the colored Lazr fingers? What does this tell us about what makes LASER light special?

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Even though the LASER was split up into its components, they were all red. Unlike the red Lazr finger, which has multiple colors in its spectrum, even though it looks red, you can see only red in the spectrum of the laser. Hence, the LASER pointer consists of only one color light, i.e. only one wavelength. This is called monochromatic light.