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154 lines
7.1 KiB
HTML
154 lines
7.1 KiB
HTML
<!DOCTYPE html>
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<title>Infrared</title>
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<article id="content" role="main">
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<h1 class="center">Infrared</h1>
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<p>Infrared (also called IR) is in the middle of the <a href="electromagnetic-spectrum.html">electromagnetic spectrum</a>:</p>
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<div class="script" style="height: 260px;">
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images/em-spectrum.js
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</div>
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<p>The wavelength of infrared goes from 1 millimeter to far red light at 750 nanometers (750 billionths of a meter).</p>
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<h2>Invisible</h2>
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<p>Infrared is invisible (under special conditions our eyes can see a little into infrared but not well).</p>
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<p class="center"><img src="images/light-spectrum.svg" alt="light spectrum" height="154" width="498"><br></p>
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<p>So why do hot things <b>glow</b>? Because they are also sending out light:</p>
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<ul>
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<li>warm objects don't glow, but you can feel the heat coming from them</li>
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<li>hot objects glow red because they send out low energy light as well</li>
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<li>very hot objects glow blue-white as they send out a large range of light</li>
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</ul>
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<p class="center"><img src="images/hot-bar.jpg" alt="hot glowing bar" height="189" width="360"></p>
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<p>(Note: flames can also have special colors due to chemistry, for example blue and violet flames are due to carbon and hydrogen.)</p>
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<h2>Heat</h2>
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<p>We all think that infrared is for heat, but <b>all electromagnetic radiation can heat</b>. For example <a href="waves-radio-microwave.html">microwave</a> ovens can cook, laser cutters can cut through steel using <a href="light.html">light</a>, and so on.</p>
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<p>But infrared does heat food nicely:</p>
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<p class="center"><img src="images/casserole.jpg" alt="chicken casserole" height="213" width="360"></p>
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<p>Typically about 2000 Watts of power is used:</p>
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<ul>
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<li>the heat gets absorbed by the food at the surface</li>
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<li>that heat then gets conducted slowly through the rest of the food</li></ul>
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<h2>Viewing Heat</h2>
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<p>Special cameras can "see" infrared radiation, and also measure temperatures.</p>
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<p class="center"><img src="images/infrared-image.jpg" alt="infrared image" height="209" width="360"></p>
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<p><br>
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We can see the image because the different wavelengths of infrared are changed into visible light by the electronics inside the camera. We call them "false color" images.</p>
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<h2>Security</h2>
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<p style="float:right; margin: 0 0 5px 10px;"><img src="images/infrared-security.svg" alt="infrared security" height="" width=""><br>
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<i>(the beams are really<br>
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invisible)</i></p>
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<p>Security systems use infrared detectors to sense when people enter a room.</p>
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<p>And invisible infrared beams get interrupted when someone walks through them, which creates an alarm. (They are low power beams, so people don't get cooked!)</p>
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<p>And infrared security cameras can "see" in the dark.</p>
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<h2>Fiber Optics</h2>
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<p>Infrared, and <a href="light.html">light</a>, can be sent along fiber optic cables, carrying information coded into the wavelength.</p>
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<p>If the signal weakens it can be boosted using special equipment.</p>
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<p class="center"><img src="images/fiber-optic.jpg" alt="fiber optic" height="242" width="360"><br>
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Fiber optic cables</p>
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<p>The light stays inside because of a special property of <a href="refraction.html">refraction</a>: when the refractive index is lower on the outside, and the angle is not too steep, the light beam has <b>total internal reflection</b> on the inside:</p>
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<p class="center"><img src="images/fiber-optic-bounce.svg" alt="fiber optic bounce" height="118" width="453"><br>
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Light bounces off the walls inside the cable</p>
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<p>Fiber optics are <b>better than electrical wires</b> which get "noise" (undesirable signals that distort or interfere with the original) from power lines, TV/Radio stations, etc.</p>
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<p>Also, many different signals can be sent on the same optic fiber cable (using different wave lengths for example.)</p>
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<h2>Remote Controls</h2>
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<p>Many remote controls use infrared at 940 nanometers to send signals to TVs and other devices. The digital signal is sent by little flashes of infrared.</p>
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<p class="center"><img src="images/ir-remote-controls.jpg" alt="ir remote controls" height="155" width="360"></p>
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<p>The signal behaves like light so cannot go through solid things, but can be reflected off some surfaces.</p>
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<p>Try seeing if your remote can control the TV by reflecting off a mirror, glass or wall!</p>
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<p> </p>
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<div class="questions">17750, 17756</div>
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<div class="related">
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<a href="electromagnetic-spectrum.html">Electromagnetic Spectrum</a>
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<a href="index.html">Physics Index</a>
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