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interference</a></label></li><li class=""><label style="padding-left: 14px"><i class="fa fa-fw"></i><a href="../1124/thin-film-interference.html">Thin film interference</a></label></li></ul></li></ul></div> <div class="hidden-xs hidden-sm"> <button class="btn btn-default btn-block text-xs-center" data-toggle="modal" data-target="#modal-feedback" style="margin-bottom: 10px"><i class="fa fa-send"></i> Feedback</button> </div> </div> <div class="col-md-9" id="main-column"> <h1 class="page_title"> Light <a href="#" class="mark-page-favorite pull-right" data-pid="284" title="Mark as favorite" onclick="return false;"><i class="fa fa-star-o"></i></a> </h1> <ol class="breadcrumb"> <li><a href="../../../physics.html"><i class="fa fa-home"></i></a><i class="fa fa-fw fa-chevron-right divider"></i></li><li><a href="../444/oscillations-and-waves.html">Oscillations and waves</a><i class="fa fa-fw fa-chevron-right divider"></i></li><li><a href="../278/waves.html">Waves</a><i class="fa fa-fw fa-chevron-right divider"></i></li><li><span class="gray">Light</span></li> <span class="pull-right" style="color: #555" title="Suggested study time: 20 minutes"><i class="fa fa-clock-o"></i> 20'</span> </ol> <article id="main-article"> <p><img alt="" src="../../waves/light.jpg" style="float: left; width: 250px; height: 167px;">To fully understand the propagation of electromagnetic waves, you will need to study the next section on electromagnetism. However, it is enough to use what we know about other waves to understand the wave nature of light.</p> <hr class="hidden-separator"> <div class="panel panel-turquoise panel-has-colored-body"> <div class="panel-heading"> <div> <p>Key Concepts</p> </div> </div> <div class="panel-body"> <div> <div class="panel panel-turquoise panel-has-colored-body panel-has-border"> <div class="panel-heading"><a class="expander" href="#"><span class="fa fa-plus"></span></a> <div> <p>Electromagnetic spectrum</p> </div> </div> <div class="panel-body"> <div> <p>Light is part of the electromagnetic spectrum. Electromagnetic waves are disturbances in electric and magnetic fields. They are not mechanical waves so require no medium.</p> <div class="video-embed vimeo"><iframe allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen="" mozallowfullscreen="" webkitallowfullscreen="" height="420" width="100%" src="https://player.vimeo.com/video/171536735"></iframe></div> </div> </div> <div class="panel-footer"> <div> <p> </p> </div> </div> </div> <div class="panel panel-turquoise panel-has-colored-body panel-has-border panel-expandable"> <div class="panel-heading"><a class="expander" href="#"><span class="fa fa-plus"></span></a> <div> <p>Reflection</p> </div> </div> <div class="panel-body"> <div> <p>When light strikes the boundary between two media, part of it reflects.</p> <div class="video-embed vimeo"><iframe allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen="" mozallowfullscreen="" webkitallowfullscreen="" height="420" width="100%" src="https://player.vimeo.com/video/171539930"></iframe></div> </div> </div> <div class="panel-footer"> <div> <p> </p> </div> </div> </div> <div class="panel panel-turquoise panel-has-colored-body panel-has-border panel-expandable"> <div class="panel-heading"><a class="expander" href="#"><span class="fa fa-plus"></span></a> <div> <p>Refraction </p> </div> </div> <div class="panel-body"> <div> <p>While part of the light reflects, the remainder will refract into the medium as it changes speed and direction. The angle of refraction can be calculated by Snell's law, where <em>n</em> is the refractive index (ratio of the speed of light to its speed in the material).</p> <p style="text-align: center;"><span class="math-tex">\({\sin i \over \sin r}={n_2 \over n_1}\)</span></p> <p>NB: <em>n<sub>1</sub></em> is often equal to 1, as light usually enters a medium from air (where its speed is equal to that in a vacuum).</p> <div class="video-embed vimeo"><iframe allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen="" mozallowfullscreen="" webkitallowfullscreen="" height="420" width="100%" src="https://player.vimeo.com/video/171885174"></iframe></div> </div> </div> <div class="panel-footer"> <div> <p> </p> </div> </div> </div> <div class="panel panel-turquoise panel-has-colored-body panel-has-border panel-expandable"> <div class="panel-heading"><a class="expander" href="#"><span class="fa fa-plus"></span></a> <div> <p>Total internal reflection</p> </div> </div> <div class="panel-body"> <div> <p>Total internal reflection occurs when light travels into a less optically dense medium at an angle of incidence that exceeds the critical angle - <strong>all</strong> of it reflects.</p> <div class="video-embed vimeo"><iframe allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen="" mozallowfullscreen="" webkitallowfullscreen="" height="420" width="100%" src="https://player.vimeo.com/video/171884451"></iframe></div> </div> </div> <div class="panel-footer"> <div> <p> </p> </div> </div> </div> </div> </div> <div class="panel-footer"> <div> </div> </div> </div> <div class="panel panel-has-colored-body panel-yellow"> <div class="panel-heading"><a class="expander" href="#"><span class="fa fa-plus"></span></a> <div> <p>Essentials</p> </div> </div> <div class="panel-body"> <div> <div class="panel panel-has-colored-body panel-has-border panel-yellow"> <div class="panel-heading"><a class="expander" href="#"><span class="fa fa-plus"></span></a> <div> <p>Dispersion </p> </div> </div> <div class="panel-body"> <div> <p>The refractive index of glass depends on the wavelength (colour) of light. It is largest for violet light, but a useful reminder can also be "blue bends best" (rather than red!).</p> <p>White light disperses into all the colours of the visible light spectrum when it passes through a prism.</p> <div class="video-embed vimeo"><iframe allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen="" mozallowfullscreen="" webkitallowfullscreen="" height="420" width="100%" src="https://player.vimeo.com/video/171886201"></iframe></div> </div> </div> <div class="panel-footer"> <div> <p> </p> </div> </div> </div> <div class="panel panel-has-colored-body panel-has-border panel-yellow panel-expandable"> <div class="panel-heading"><a class="expander" href="#"><span class="fa fa-plus"></span></a> <div> <p>Interference</p> </div> </div> <div class="panel-body"> <div> <p>Two <em>coherent </em>sources of light (equal frequencies) will interfere where they overlap. This causes the formation of bright and dark patches called <em>fringes.</em></p> <div class="video-embed vimeo"><iframe allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen="" mozallowfullscreen="" webkitallowfullscreen="" height="420" width="100%" src="https://player.vimeo.com/video/171911897"></iframe></div> </div> </div> <div class="panel-footer"> <div> <p> </p> </div> </div> </div> <div class="panel panel-has-colored-body panel-has-border panel-yellow panel-expandable"> <div class="panel-heading"><a class="expander" href="#"><span class="fa fa-plus"></span></a> <div> <p>Diffraction</p> </div> </div> <div class="panel-body"> <div> <p>Light spreads out when it passes through a slit. The angle of spreading depends on the width of the slit:</p> <p style="text-align: center;"><span class="math-tex">\(d\sin \theta=\lambda\)</span></p> <div class="video-embed vimeo"><iframe allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen="" mozallowfullscreen="" webkitallowfullscreen="" height="420" width="100%" src="https://player.vimeo.com/video/171883546"></iframe></div> </div> </div> <div class="panel-footer"> <div> <p> </p> </div> </div> </div> </div> </div> </div> <div class="panel panel-has-colored-body panel-green"> <div class="panel-heading"><a class="expander" href="#"><span class="fa fa-plus"></span></a> <div> <p>Test Yourself</p> </div> </div> <div class="panel-body"> <div> <p><i>Use quizzes to practise application of theory.</i></p> <br><a class="btn btn-primary btn-block text-center" data-toggle="modal" href="#5338bc91"><i class="fa fa-play"></i> START QUIZ!</a><div class="modal fade modal-slide-quiz" id="5338bc91"> <div class="modal-dialog" style="width: 95vw; max-width: 960px"> <div class="modal-content"> <div class="modal-header slide-quiz-title"> <h4 class="modal-title" style="width: 100%;"> Light waves <strong class="q-number pull-right"> <span class="counter">1</span>/<span class="total">1</span> </strong> </h4> </div> <div class="modal-body p-xs-3"> <div class="slide-quiz" data-stats="6-92-284" style="opacity: 0"> <div class="exercise shadow-bottom"><div class="q-question"><p>Which of the following lists includes the em waves in order of increasing wavelength</p></div><div class="q-answer"><p><label class="radio"><input class="c" type="radio"> X, UV, light, IR, radio</label></p><p><label class="radio"><input type="radio"> X, IR light, UV, radio</label></p><p><label class="radio"><input type="radio"> IR, radio, light, X, UV</label></p><p><label class="radio"><input type="radio"> radio, IR, light, UV, X</label></p></div><div class="q-explanation"><p>that´s just the way it is</p></div><div class="slide-q-actions"><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div><div class="exercise shadow-bottom"><div class="q-question"><p>The velocity of em waves in a vacuum:</p></div><div class="q-answer"><p><label class="radio"><input type="radio"> is greater for X rays than radio waves</label></p><p><label class="radio"><input type="radio"> is different for different colours</label></p><p><label class="radio"><input class="c" type="radio"> is the same for all types</label></p><p><label class="radio"><input type="radio"> is greater for radio waves than X rays</label></p></div><div class="q-explanation"><p>The velocity of em radiation is 3 x 10<sup>8</sup> ms<sup>-1</sup></p></div><div class="slide-q-actions"><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div><div class="exercise shadow-bottom"><div class="q-question"><p>The electric and magnetic fields in an em wave are:</p></div><div class="q-answer"><p><label class="radio"><input type="radio"> perpendicular to each other and parallel to the direction of propagation</label></p><p><label class="radio"><input class="c" type="radio"> perpendicular to each other and the direction of propagation</label></p><p><label class="radio"><input type="radio"> perpendicular to each other and the electric field is perpendicular to the direction of propagation</label></p><p><label class="radio"><input type="radio"> perpendicular to each other and the magnetic wave is perpendicular to the direction of propagation</label></p></div><div class="q-explanation"><p style="text-align: center;"><img alt="" src="../../emwavea1.jpg" style="width: 166px; height: 144px;"></p></div><div class="slide-q-actions"><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div><div class="exercise shadow-bottom"><div class="q-question"><p>A ray of light is reflected off an inclined mirror as shown.</p><p style="text-align: center;"><img alt="" src="../../reflectionq1(1).jpg" style="width: 492px; height: 180px;"></p><p>The angle of reflection is</p></div><div class="q-answer"><p><label class="radio"><input type="radio"> 60°</label></p><p><label class="radio"><input type="radio"> 30°</label></p><p><label class="radio"><input type="radio"> 40°</label></p><p><label class="radio"><input class="c" type="radio"> 50°</label></p></div><div class="q-explanation"><p style="text-align: center;"><img alt="" src="../../refllighta1.jpg" style="width: 340px; height: 237px;"></p></div><div class="slide-q-actions"><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div><div class="exercise shadow-bottom"><div class="q-question"><p>A ray of light is reflected by two mirrors as shown</p><p style="text-align: center;"><img alt="" src="../../lightrefq3.jpg" style="width: 273px; height: 253px;"></p><p>What is the angle of reflection at the second mirror?</p></div><div class="q-answer"><p><label class="radio"><input type="radio"> 30°</label></p><p><label class="radio"><input type="radio"> 10°</label></p><p><label class="radio"><input class="c" type="radio"> 20°</label></p><p><label class="radio"><input type="radio"> 40°</label></p></div><div class="q-explanation"><p style="text-align: center;"><img alt="" src="../../lightrefla3.jpg" style="width: 289px; height: 240px;"></p></div><div class="slide-q-actions"><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div><div class="exercise shadow-bottom"><div class="q-question"><p>The diagram shows a ray of light incident on 4 blocks.</p><p style="text-align: center;"><img alt="" height="323" src="../../screen-shot-2016-09-02-at-07.03.30.png" width="152"></p><p>Which block is glass?</p></div><div class="q-answer"><p><label class="radio" style=" float: left; margin-right: 40px; "><input class="c" type="radio"> B</label></p><p><label class="radio" style=" float: left; margin-right: 40px; "><input type="radio"> A</label></p><p><label class="radio" style=" float: left; margin-right: 40px; "><input type="radio"> C</label></p><p><label class="radio" style=" float: left; margin-right: 40px; "><input type="radio"> D</label></p></div><div class="q-explanation"><p>Refractive index is 1.5. C and D refract too much A not enough</p></div><div class="slide-q-actions"><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div><div class="exercise shadow-bottom"><div class="q-question"><p>The diagram shows 4 possible/impossible paths for light travelling through a glass block</p><p style="text-align: center;"><img alt="" src="../../reflightq2.jpg" style="width: 280px; height: 264px;"></p><p>The red line shows the path of red light. Which is the path of blue?</p></div><div class="q-answer"><p><label class="radio" style=" float: left; margin-right: 40px; "><input class="c" type="radio"> C</label></p><p><label class="radio" style=" float: left; margin-right: 40px; "><input type="radio"> A</label></p><p><label class="radio" style=" float: left; margin-right: 40px; "><input type="radio"> B</label></p><p><label class="radio" style=" float: left; margin-right: 40px; "><input type="radio"> D</label></p></div><div class="q-explanation"><p>Blue light is refracted more but the emerging ray is still parallel to the incident ray.</p></div><div class="slide-q-actions"><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div><div class="exercise shadow-bottom"><div class="q-question"><p>A ray of light passes across a boundary as shown.</p><p style="text-align: center;"><img alt="" src="../../reflightq3.jpg" style="width: 192px; height: 237px;"></p><p>What is the refractive index of the boundary?</p><p style="text-align: center"><iframe height="420" scrolling="no" src="http://web2.0calc.com/widgets/horizontal/?options=%7B%22angular%22%3A%22deg%22%2C%22options%22%3A%22hide%22%2C%22menu%22%3A%22show%22%7D" style="border: 1px solid silver; " width="590"></iframe><br><a href="http://web2.0calc.com/">Web 2.0 scientific calculator</a></p></div><div class="q-answer"><p><label class="radio"><input class="c" type="radio"> 1.46</label></p><p><label class="radio"><input type="radio"> 1.33</label></p><p><label class="radio"><input type="radio"> 1.50</label></p><p><label class="radio"><input type="radio"> 0.68</label></p></div><div class="q-explanation"><p>sini/sinr</p></div><div class="slide-q-actions"><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div><div class="exercise shadow-bottom"><div class="q-question"><p>A ray of light is incident on a glass - air boundary at 45° as shown</p><p style="text-align: center;"><img alt="" src="../../lighttirq1.jpg" style="width: 248px; height: 228px;"></p><p>Which ray represents the light after hitting the boundary?</p></div><div class="q-answer"><p><label class="radio" style=" float: left; margin-right: 40px; "><input class="c" type="radio"> A</label></p><p><label class="radio" style=" float: left; margin-right: 40px; "><input type="radio"> B</label></p><p><label class="radio" style=" float: left; margin-right: 40px; "><input type="radio"> C</label></p><p><label class="radio" style=" float: left; margin-right: 40px; "><input type="radio"> D</label></p></div><div class="q-explanation"><p>The critical angle for glass is 43.8° any angle above this will cause total internal reflection</p></div><div class="slide-q-actions"><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div><div class="exercise shadow-bottom"><div class="q-question"><p>Water has refractive index 1.3. What is the critical angle for water?</p><p style="text-align: center"><iframe height="420" scrolling="no" src="http://web2.0calc.com/widgets/horizontal/?options=%7B%22angular%22%3A%22deg%22%2C%22options%22%3A%22hide%22%2C%22menu%22%3A%22show%22%7D" style="border: 1px solid silver; " width="590"></iframe><br><a href="http://web2.0calc.com/">Web 2.0 scientific calculator</a></p></div><div class="q-answer"><p><label class="radio"><input class="c" type="radio"> 50.3°</label></p><p><label class="radio"><input type="radio"> 90°</label></p><p><label class="radio"><input type="radio"> 45°</label></p><p><label class="radio"><input type="radio"> 43.8°</label></p></div><div class="q-explanation"><p>sin<sup>-1</sup>(1/n)</p></div><div class="slide-q-actions"><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div><div class="exercise shadow-bottom"><div class="q-question"><p>Visible diffraction effects can be observed when light passes through an opening of width</p></div><div class="q-answer"><p><label class="radio"><input type="radio"> 1 m</label></p><p><label class="radio"><input type="radio"> 1 nm</label></p><p><label class="radio"><input type="radio"> 1cm</label></p><p><label class="radio"><input class="c" type="radio"> 0.01 mm</label></p></div><div class="q-explanation"><p>There would be diffraction for 1 nm but the amount of light passing would be too little to see</p></div><div class="slide-q-actions"><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div><div class="exercise shadow-bottom"><div class="q-question"><p>The diagram represents the interference of light passing through two slits.</p><p style="text-align: center;"><img alt="" src="../../intlightq1.jpg" style="width: 344px; height: 188px;"></p><p>The path difference between the waves at point A is:</p></div><div class="q-answer"><p><label class="radio"><input type="radio"> 0.5λ</label></p><p><label class="radio"><input type="radio"> 2.5λ</label></p><p><label class="radio"><input class="c" type="radio"> 1.5λ</label></p><p><label class="radio"><input type="radio"> 1.0λ</label></p></div><div class="q-explanation"><p>The second region of destructive interference</p></div><div class="slide-q-actions"><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div><div class="exercise shadow-bottom"><div class="q-question"><p>The diagram represents the interference of light passing through two slits.</p><p style="text-align: center;"><img alt="" src="../../intlightq1.jpg" style="width: 344px; height: 188px;"></p><p>The phase difference between the waves at point A is:</p></div><div class="q-answer"><p><label class="radio"><input type="radio"> π/2</label></p><p><label class="radio"><input class="c" type="radio"> π</label></p><p><label class="radio"><input type="radio"> 3π/2</label></p><p><label class="radio"><input type="radio"> 2π</label></p></div><div class="q-explanation"><p>Out of phase so phase difference is π</p></div><div class="slide-q-actions"><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div> </div> </div> <div class="modal-footer slide-quiz-actions"> <div class=""> <div class="pull-left pull-xs-none mb-xs-3"> <button class="btn btn-default d-xs-none btn-prev"> <i class="fa fa-arrow-left"></i> Prev </button> </div> <div class="pull-right pull-xs-none"> <button class="btn btn-success btn-xs-block text-xs-center btn-results" style="display: none"> <i class="fa fa-bar-chart"></i> Check Results </button> <button class="btn btn-default d-xs-none btn-next"> Next <i class="fa fa-arrow-right"></i> </button> <button class="btn btn-default btn-xs-block text-xs-center btn-close" data-dismiss="modal" style="display: none"> Close </button> </div> </div> </div> </div> </div></div> </div> </div> <div class="panel-footer"> <div> </div> </div> </div> <div class="panel panel-has-colored-body panel-default"> <div class="panel-heading"><a class="expander" href="#"><span class="fa fa-plus"></span></a> <div> <p>Exam-style Questions</p> </div> </div> <div class="panel-body"> <div><em>Online tutorials to help you solve original problems</em></div> <div> <div class="panel panel-has-colored-body panel-has-border panel-default"> <div class="panel-heading"><a class="expander" href="#"><span class="fa fa-plus"></span></a> <div> <p>Question 1</p> </div> </div> <div class="panel-body"> <div> <iframe src="../../../pages/subjects/page-networks/index-55.htm?ticket=082b9c9c4ae3624d&network-id=51&sitename=physics&subject-id=6&fullscreen=false&node=start" style="width: 100%; height: 640px; max-height: 80vh" id="network-51" class="network-viewer"></iframe> </div> </div> <div class="panel-footer"> <div> <p> </p> </div> </div> </div> <div class="panel 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