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418 lines
17 KiB
HTML
418 lines
17 KiB
HTML
<!DOCTYPE html>
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<html lang="en"><!-- #BeginTemplate "/Templates/Advanced.dwt" --><!-- DW6 -->
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<!-- Mirrored from www.mathsisfun.com/physics/light.html by HTTrack Website Copier/3.x [XR&CO'2014], Sat, 29 Oct 2022 00:51:10 GMT -->
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<meta http-equiv="content-type" content="text/html; charset=UTF-8">
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<!-- #BeginEditable "doctitle" -->
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<title>Light</title>
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<meta name="description" content="Math explained in easy language, plus puzzles, games, quizzes, videos and worksheets. For K-12 kids, teachers and parents.">
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<script>reSpell=[["meter","metre"],["meters","metres"],["nanometer","nanometre"],["color","colour"],["colors","colours"],["fiber","fibre"]];</script>
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<div id="hdr"></div>
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<div id="cookOK"></div>
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<div class="mid">
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<nav>
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<div id="menuWide" class="menu"></div>
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<div id="logo"><a href="../index.html"><img src="../images/style/logo-adv.svg" alt="Math is Fun Advanced"></a></div>
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<div id="extra"></div>
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<article id="content" role="main">
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<!-- #BeginEditable "Body" -->
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<h1 class="center">Light</h1>
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<p>We use light to see!</p>
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<!-- <p>Plants also use light for photosynthesis (the energy from light helps them convert chemicals).</p>
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--> <p>Visible light is the part of the <a href="electromagnetic-spectrum.html">electromagnetic spectrum</a> that our eyes can see:</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>It is only a small part of the full spectrum, isn't it?</p>
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<h2>Visible Spectrum</h2>
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<p class="words"><b>Visible Light</b>: the wavelengths that are visible to most human eyes.</p>
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<p class="center">The main colors, in order, go "Roy G Bv": <b>R</b>ed <b>O</b>range <b>Y</b>ellow <b>G</b>reen <b>B</b>lue <b>V</b>iolet<br>
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<img src="images/light-spectrum.svg" alt="light spectrum" height="154" width="498"></p>
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<p class="center">As we see on this beautiful rainbow:<br>
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<img src="images/rainbow.jpg" alt="rainbow" height="209" width="360"></p>
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<h2>Wavelength</h2>
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<p class="center"><img src="images/wavelength.svg" alt="wavelength" height="153" width="404"></p>
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<p>Light has a <a href="../definitions/wavelength.html">wavelength</a> of about 380 nm to 750 nm, depending on color.</p>
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<div class="def">
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<p><b>nm</b> means <b>nanometer</b>, one billionth of a meter.</p>
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<p>Example: <b><span style="color:red;">red light</span></b> has a wavelength of about 700 billionths of a meter (just less than one-millionth of a meter). Small!</p></div>
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<p>Definitions vary, but here is a rough guide:</p>
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<div class="simple">
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<table style="border: 0; margin:auto;">
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<tbody>
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<tr>
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<th>Color</th>
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<th align="center">Wavelength Range (nm)</th>
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</tr>
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<tr>
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<td>Red </td>
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<td style="text-align:center;">620–750</td>
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</tr>
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<tr>
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<td>Orange</td>
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<td style="text-align:center;">590–620</td>
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</tr>
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<tr>
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<td>Yellow</td>
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<td style="text-align:center;">570–590</td>
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</tr>
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<tr>
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<td>Green</td>
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<td style="text-align:center;">495–570</td>
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</tr>
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<tr>
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<td>Blue</td>
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<td style="text-align:center;">450–495</td>
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</tr>
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<tr>
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<td>Violet</td>
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<td style="text-align:center;">380–450</td>
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</tr>
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</tbody></table>
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</div>
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<h2>Frequency</h2>
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<p>The <b>frequency</b> of red light is about 400 THz (and for violet is about 800 THz)</p>
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<div class="def">
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<p><b>THz</b> means <b>teraHertz</b>, a trillion cycles per second</p>
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<p>So red light vibrates at about 400 million <i>million</i> cycles per second. Fast!</p>
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</div>
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<p>Higher frequency (with shorter wavelength) has more energy:</p>
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<ul>
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<li>Red light has lower frequency, longer wavelength and less energy</li>
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<li>Blue light has higher frequency, shorter wavelength and more energy</li>
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</ul>
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<p class="center"><img src="images/light-red-blue.svg" alt="red lower energy, blue higher energy" height="245" width="338"></p>
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<h2>Speed of Light</h2>
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<p>Light travels at almost <b>300,000,000 meters per second</b> (to be exact: 299,792,458 meters per second) in a vacuum.</p>
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<p>That is <b>300 million meters</b> every second, or:</p>
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<ul>
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<li>3 × 10<sup>8</sup> m/s</li>
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<li>300,000 km/s</li>
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<li>186,000 miles per second</li>
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</ul>
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<p>At that speed light travels:</p>
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<div class="simple">
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<table style="border: 0; margin:auto;">
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<tbody>
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<tr>
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<th>Distance</th>
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<th> </th>
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<th>Time</th>
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</tr>
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<tr>
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<td style="text-align:center;">1 meter</td>
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<td style="text-align:center;">in</td>
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<td style="text-align:center;">3.3 ns (3.3 billionths of a second)</td>
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</tr>
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<tr>
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<td style="text-align:center;">Around the Earth's equator</td>
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<td style="text-align:center;">in</td>
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<td style="text-align:center;">134 ms (134 thousandths of a second)</td>
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</tr>
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<tr>
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<td style="text-align:center;">From Earth to Moon</td>
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<td style="text-align:center;">in</td>
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<td style="text-align:center;">1.3 s </td>
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</tr>
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<tr>
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<td style="text-align:center;">Surface of Sun to Earth</td>
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<td style="text-align:center;">in</td>
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<td style="text-align:center;">about 8 minutes </td>
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</tr>
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</tbody></table>
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</div>
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<p>It is so fast, but still takes about 8 minutes from the surface of the Sun to the Earth.</p>
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<p>The symbol for this speed is <span class="times">c</span>:</p>
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<p class="center large"><b>c</b> ≈ 300,000,000 m/s</p>
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<h2>Light Can Travel Slower</h2>
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<p>We really shouldn't call it the speed of <b>light</b>, firstly because it applies to the whole electromagnetic spectrum, and gravity waves, and more. Maybe we could call it "Max Speed"!</p>
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<p>But also because light <b>only travels that speed in a vacuum</b>! It can travel slower ...</p>
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<div class="simple">
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<table style="border: 0; margin:auto;">
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<tbody>
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<tr>
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<th>Medium</th>
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<th align="center"><b>Speed</b><br>
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million m/s</th>
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</tr>
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<tr>
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<td>Vacuum</td>
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<td style="text-align:center;">299.8</td>
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</tr>
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<tr>
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<td>Air</td>
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<td style="text-align:center;">299.7</td>
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</tr>
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<tr>
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<td>Ice</td>
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<td style="text-align:center;">228</td>
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</tr>
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<tr>
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<td>Water</td>
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<td style="text-align:center;">225</td>
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</tr>
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<tr>
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<td>Ethanol</td>
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<td style="text-align:center;">220</td>
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</tr>
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<tr>
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<td>Glass</td>
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<td style="text-align:center;">205</td>
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</tr>
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<tr>
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<td>Olive oil</td>
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<td style="text-align:center;">204</td>
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</tr>
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<tr>
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<td>Diamond</td>
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<td style="text-align:center;">123</td>
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</tr>
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</tbody></table>
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</div>
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<h2>Wavelength and Frequency are Linked</h2>
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<p>The Wavelength and Frequency are related:</p>
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<p class="large center"><span style="color:blue">Frequency</span> = <span class="intbl">
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<em>Velocity</em>
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<span style="color:orange">Wavelength</span></span></p>
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<p class="large center"><span style="color:orange">Wavelength</span> = <span class="intbl">
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<em>Velocity</em>
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<strong><span style="color:blue">Frequency</span></strong>
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</span></p>
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<p>Assuming the light is in a vacuum, the velocity is the speed of light: <b>3 × 10<sup>8</sup> m/s</b></p>
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<p>Let's try a simple example (in this case <i>not</i> a wavelength of light):</p>
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<div class="example">
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<h3>Imagine a very long wavelength of 75,000 km</h3>
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<p class="center"><img src="images/wavelength-vs-frequency.svg" alt="wavelength vs frequency" height="151" width="404"></p>
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<p class="large center"><span style="color:blue">Frequency</span> = <span class="intbl">
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<em>300,000 km/s</em>
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<strong><span style="color:orange">75,000 km</span></strong></span></p>
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<p class="large center">= <span style="color:blue">4 /s</span></p>
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<p class="large center">= <span style="color:blue">4 Hz</span></p>
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<p>We can fit <b>4</b> of those wavelengths in 300,000 km, so it vibrates 4 times in 1 second.</p>
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<p>So the frequency is 4 Hz (4 per second)</p>
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<p>Or, the other way around, if we know it vibrates 4 times a second we can calculate its wavelength:</p>
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<p class="large center"><span style="color:orange">Wavelength</span> = <span class="intbl">
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<em>300,000 km/s</em>
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<strong><span style="color:blue">4 /s</span></strong>
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</span></p>
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<p class="large center"> = <span style="color:orange">75,000 km</span></p>
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</div>
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<div class="example">
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<h3>Example: Blue light has a wavelength of about 480 nm (480 × 10<sup>-9</sup> m)</h3>
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<p>So the frequency is:</p>
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<p class="large center"><span style="color:blue">Frequency</span> = <span class="intbl">
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<em>3 × 10<sup>8</sup> m/s</em>
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<strong><span style="color:orange">480 × 10<sup>-9</sup> m</span></strong></span></p>
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<p class="large center">= <span style="color:blue">6.25 × 10<sup>14</sup> /s</span></p>
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<p class="large center">= <span style="color:blue">6.25 × 10<sup>14</sup> Hz</span></p>
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<p>Which is 625 TeraHertz</p></div>
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<h2>Light Travels in Straight Lines</h2>
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<p>Light <b>travels in a straight line</b> until its hits something, or it's path is changed by different densities, or by gravity.</p>
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<p class="center"><img src="images/light-beams-forest.jpg" alt="light beams forest" height="244" width="600"><br>
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Light from the Sun streams across the road.<br>
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The shadows also show that light travels in straight lines.</p>
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<p class="center"><img src="images/light-beam.jpg" alt="light beam" height="191" width="594"><br>
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This light spreads out a little and is scattered by the atmosphere.</p>
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<p class="center"><img src="images/laser-beams.jpg" alt="laser beams" height="262" width="600"><br>
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Laser beams making straight lines.</p>
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<p> </p>
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<p style="float:right; margin: 0 0 5px 10px;"><img src="images/refraction-plastic-block.jpg" alt="refraction plastic block" height="227" width="240"></p>
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<h2>Wave</h2>
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<p>Light behaves as a wave, so it can:</p>
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<ul>
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<li><a href="reflection.html">reflect</a> (bounce off),</li>
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<li>scatter (bounce off in all directions),</li>
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<li><a href="refraction.html">refract</a> (change speed and direction)</li>
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<li><a href="diffraction.html">diffract</a> (spread out past an opening)</li>
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<li>transmit (pass straight through)</li>
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<li>or get absorbed</li>
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</ul>
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<h2><span class="center">Photons</span></h2>
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<p><span class="center">Light <b>also</b></span> behaves as packets of energy called <b>Photons</b>.</p>
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<ul>
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<li>We can measure a photon's position and momentum.</li>
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<li>Photons have no mass, but each photon has an amount of energy based on its frequency (number of vibrations per second)</li>
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<li>Each photon has a wavelength</li>
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</ul>
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<p> </p>
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<p>So it is like a <b>particle</b> and also like a <b>wave</b>. This is called the "wave-particle duality".</p>
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<p style="float:left; margin: 0 10px 5px 0;"><img src="images/einstein.jpg" alt="einstein" height="217" width="200"></p>
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<p> </p>
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<p>Einstein wrote:</p>
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<p><i>"It seems as though we must use sometimes the one theory and sometimes the other, while at times we may use either."</i></p>
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<div style="clear:both"></div>
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<h2>Intensity</h2>
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<p style="float:right; margin: 0 0 5px 10px;"> </p>
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<p>Intensity is <b>power per area</b>, usually in Watts per square meter:</p>
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<p class="center larger">Intensity = W/m<sup>2</sup></p>
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<div class="example">
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<h3>Example: Sun on a small 100 square meter house</h3>
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<p>About 150 to 300 <b>watts</b> of energy are received from the Sun <b>per square meter</b>.</p>
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<p>Let's choose the smaller number:</p>
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<p class="center large">Intensity = 150 W/m<sup>2</sup></p>
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<p>How much Power is that over the whole roof?</p>
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<p class="center large">Power = 150 W/m<sup>2</sup> × 100 m<sup>2</sup></p>
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<p class="center large">Power = 15,000 W</p>
|
||
<p>So a small house gets about <b>15 kilowatts</b> on it's roof, which is <b>several times more than a household uses.</b></p>
|
||
<p>But that is only while the Sun shines, and only about 20% can be captured by typical solar panels</p>
|
||
<p>But that is still lots of energy from the Sun.</p>
|
||
</div>
|
||
|
||
|
||
<h2>Inverse Square</h2>
|
||
<p style="float:left; margin: 0 10px 5px 0;"><img src="../algebra/images/proportion-inverse-square.svg" alt="brightness decreases by the square of the distance" height="" width=""></p>
|
||
<p>
|
||
|
||
</p>
|
||
<p class="large"><span><b>Inverse Square</b></span>: when one value <b>decreases</b> as the square of the other value.</p>
|
||
<div style="clear:both">
|
||
|
||
</div>
|
||
|
||
|
||
<div class="example">
|
||
|
||
<h3>Example: light and distance</h3>
|
||
<p>The further away we are from a light, the less bright it is.</p>
|
||
<p class="center"><img src="images/inverse-square-law.svg" alt="inverse square law: distance=1 area=1 intensity=1, distance=2 area=4 intensity=0.25, distance=3 area=9 intensity=0.111..." height="265" width="480"></p>
|
||
<p>The brightness decreases as the <b>square</b> of the distance. Because the light is spreading out in all directions:</p>
|
||
<ul>
|
||
<li>the energy twice as far away is spread over 4 times the area</li>
|
||
<li>the energy 3 times as far away is spread over 9 times the area</li>
|
||
<li>etc</li>
|
||
</ul>
|
||
</div>
|
||
|
||
|
||
<h2>Polarization</h2>
|
||
|
||
<p>Light is normally free to vibrate in any direction at right angles to its path.</p>
|
||
<p>But <b>polarized</b> light vibrates in one plane only:</p>
|
||
<p class="center"><img src="images/polarized.svg" alt="unpolarized vs polarized" height="162" width="365"><br>
|
||
Light gets partly polarized when it<br>
|
||
bounces off surfaces like water or glass.</p>
|
||
<p><b>Polarizing lenses</b> can block light from that plane, to cut down on reflected light and make it easier to see into water:</p>
|
||
<p class="center"><img src="images/polarized-water.jpg" alt="polarized picture of water" height="148" width="400"><br>
|
||
<b>Without</b> and <b>with</b> a polarizing lens</p>
|
||
|
||
|
||
<h2>Fiber Optics</h2>
|
||
|
||
<p>Light, and <a href="infrared.html">infrared</a>, can be sent along fiber optic cables, carrying information coded into the wavelength.</p>
|
||
<p class="center"><img src="images/fiber-optic.jpg" alt="fiber optic" height="242" width="360"><br>
|
||
Fiber optic cables</p>
|
||
<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>
|
||
<p class="center"><img src="images/fiber-optic-bounce.svg" alt="fiber optic bounce inside" height="118" width="453"><br>
|
||
Light bounces off the walls inside the cable</p>
|
||
<p>Fiber optic cables are <b>much better than electrical wires:</b></p>
|
||
<ul>
|
||
<li>Wires get more "noise" (other signals that distort or interfere with the original) from power lines, TV, radio, lightning etc.</li>
|
||
<li>Photons have no mass so can swap between 0 and 1 quickly. Electrons have mass and are slow in comparison</li>
|
||
<li>Glass has much less resistance to light than copper does to electrical signals, so can go much further without needing a boost</li>
|
||
</ul>
|
||
<p> </p>
|
||
<div class="questions">17760, 17763, 17764, 17766, 17767, 17761, 17762, 17765, 17768, 17769</div>
|
||
|
||
<div class="related">
|
||
<a href="electromagnetic-spectrum.html">Electromagnetic Spectrum</a>
|
||
<a href="index.html">Physics Index</a>
|
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