Slide 1 / 108 Slide 2 / 108 Algebra Based Physics Electromagnetic Waves 2015-12-01 www.njctl.org Slide 3 / 108 Slide 4 / 108 Table of Contents Click on the topic to go to that section An Abridged "History" of Light · An Abridged "History" Reflection, Refraction and Dispersion of Light · of Light Diffraction and Interference of Light · Maxwell's Equations · Properties of Electromagnetic Waves · Return to Table of Contents https://www.njctl.org/video/?v=BStK2Nt3CBE https://www.njctl.org/video/?v=cYF-V8ms-yA Slide 5 / 108 Slide 6 / 108 An Abridged "History" of Light An Abridged "History" of Light In 1704, Sir Isaac Newton published "Opticks," which described The first dispute with the wave nature of light came in 1900 with light as a group of tiny particles that he called corpuscles. Max Planck's explanation of Black Body Radiation where it appeared that light was emitted only in quantized bits of energy However, certain properties of light, such as diffraction - the - like a particle. bending of light around objects - was better described by thinking of light as a wave. This theory is credited to Christiaan In 1905, Albert Einstein published a paper on the photoelectric Huygens with work done by Robert Hooke and Leonhard Euler. effect (for which he later earned his Nobel Prize) which confirmed that light came in discrete packets of energy. In 1803, Thomas Young's Double Slit Experiment definitively proved that light acted as a wave. These packets of light energy were named photons by Gilbert Lewis in 1926. Maxwell then published his four equations of electromagnetism in 1861 where he treated light as a wave. So, light was explained in the classical physics as a wave, and the new field of quantum physics brought back Newton's idea of And then came relativity and quantum mechanics...... light as a particle.
Slide 7 / 108 Slide 8 / 108 An Abridged "History" of Light An Abridged "History" of Light The final word (for now) came with the correct use of relativity You should have a feel now for how light has been the subject of and quantum theory that deals with the interaction of electrons much experimentation and dispute. Even now, people talk about with photons. the "wave-particle" duality of light (and as you go on in physics, you This branch of physics is called Quantum Electrodynamics and will in 1965, Sin-Itiro Tomonaga, Julian Schwinger and Richard see a similar behavior of elementary particles such as Feynamn received the Nobel Prize for this work. Here are electrons). Feynman's words on light from his book, QED, the strange theory of light and matter: A good way to think about this is that the actual world we observe, with our senses and instruments, is way more complex and it is "I want to emphasize that light comes in this form - particles. It difficult for us to explain what is really going on. is very important to know that light behaves like particles, especially for those of you have gone to school, where you were But, for now, we will start with Newton and his corpuscle theory of probably told something about light behaving like waves. I'm light and see how it explains refraction and reflection of light. telling you the way it does behave - like particles." Slide 9 / 108 Slide 9 (Answer) / 108 1 The original wave theory of light is attributed to: 1 The original wave theory of light is attributed to: A Christian Huygens A Christian Huygens B Isaac Newton B Isaac Newton C Max Planck C Max Planck Answer D Albert Einstein D Albert Einstein A [This object is a pull tab] https://www.njctl.org/video/?v=eGK2Iagt8fw https://www.njctl.org/video/?v=eGK2Iagt8fw Slide 10 / 108 Slide 10 (Answer) / 108 2 The original particle theory of light is attributed to: 2 The original particle theory of light is attributed to: A Christian Huygens A Christian Huygens B Isaac Newton B Isaac Newton C Max Planck C Max Planck D Albert Einstein D Albert Einstein Answer B [This object is a pull tab] https://www.njctl.org/video/?v=MoZAevs4L4g https://www.njctl.org/video/?v=MoZAevs4L4g
Slide 11 / 108 Slide 11 (Answer) / 108 3 The interaction of light with matter (such as electrons) is 3 The interaction of light with matter (such as electrons) is explained by which theory? explained by which theory? A Law of Gravitation A Law of Gravitation B Coulomb's Law B Coulomb's Law C Special Relativity C Special Relativity D Quantum Electrodynamics D Quantum Electrodynamics Answer D [This object is a pull tab] https://www.njctl.org/video/?v=zqrXtoz-6F4 https://www.njctl.org/video/?v=zqrXtoz-6F4 Slide 12 / 108 Slide 13 / 108 Isaac Newton's Opticks Light is made up of tiny particles called corpuscles. Light is reflected by some surfaces, and the angle of return equals the angle of incidence. Light can be refracted - bent - as it passes from one medium Reflection, Refraction to another. and Dispersion of Light White light can be separated by a prism into many colors. But each specific color cannot be separated. All of these properties are explained nicely by the particle theory of light. Return to Table of Contents https://www.njctl.org/video/?v=imcjUchFiiQ Slide 14 / 108 Slide 15 / 108 Reflection Refraction When light transits from one media to another (air to water), the light bends. Light originating from Point P is incident on the vertical surface, m, Stick in a glass Stick in glass half The first two pictures and reflects with the same angle of air. filled with water. superimposed. The as the incident angle. image under water is The Matterhorn reflected shifted. in a lake.
Slide 16 / 108 Slide 17 / 108 Refraction Index of Refraction Some light is reflected at the interface between two The Index of Refraction, n, is a measure of how the speed and different media. Some is refracted and the angle the the wavelength of light changes when it passes from one medium refracted ray makes with the normal is called the angle of to another. The frequency of the light wave stays constant. refraction. The frequency needs to stay constant so that the waves do not Refracted Normal Normal Incident Reflected ray line line ray pile up at the interface between the two media. ray # 1 # 2 The Index of Refraction is defined as the ratio of the speed of Air (n 1 ) Air (n 2 ) light in a vacuum (c) to the speed of light in the medium (v). Water (n 2 ) Water (n 1 ) Refracted ray # 1 # 2 Reflected Incident ray ray n is the Index of Refraction and will be discussed next. Slide 18 / 108 Slide 19 / 108 Index of Refraction Index of Refraction Given that the frequency of a light wave ( ) is a ratio of its speed Dividing these equations by each other, and recognizing that the to its wavelength ( ), we have: frequency stays constant, we obtain: In a medium where the speed of light is and the wavelength is : The left term is the index of refraction of the medium, so we have: Slide 20 / 108 Slide 21 / 108 Indices of Refraction Index of Refraction Summary Here are some sample Indices The frequency of the light ray stays constant in all media. of Refraction. As n increases, the speed of light in that medium The effective speed of light in a medium other than the vacuum is decreases and the wavelength slower than the vacuum speed to the absorption and reemission of increases. the light by the molecules in the medium. The Index of Refraction also In materials other than a vacuum, the wavelength of the light ray depends on the wavelength of the increases. incident light - and that contributes to the separation of The Index of Refraction is equal to 1 in a vacuum, and is always colors in a prism. greater than 1 in other media. As light enters a new medium, it will bend towards the normal to the surface in the medium with a higher Index of Refraction.
Slide 22 / 108 Slide 22 (Answer) / 108 4 Light travels fastest: 4 Light travels fastest: A In a vacuum. A In a vacuum. B Through water. B Through water. C Through glass. C Through glass. Answer A D Through diamond. D Through diamond. [This object is a pull tab] https://www.njctl.org/video/?v=z_tWf5YDmMw https://www.njctl.org/video/?v=z_tWf5YDmMw Slide 23 / 108 Slide 23 (Answer) / 108 5 For all transparent materials, the index of refraction is: 5 For all transparent materials, the index of refraction is: A less than 1. A less than 1. B greater than 1. B greater than 1. C equal to 1. C equal to 1. Answer D depends on the material density. D depends on the material density. B [This object is a pull tab] https://www.njctl.org/video/?v=zr4B36FrKRc https://www.njctl.org/video/?v=zr4B36FrKRc Slide 24 / 108 Slide 24 (Answer) / 108 6 The Index of Refraction of diamond is 2.42. This means 6 The Index of Refraction of diamond is 2.42. This means that light travels: that light travels: A 2.42 times faster in air than it does in diamond. A 2.42 times faster in air than it does in diamond. B 2.42 times faster in diamond than it does in air. B 2.42 times faster in diamond than it does in air. C 2.42 times faster in the vacuum than it does in C 2.42 times faster in the vacuum than it does in diamond. diamond. Answer C D 2.42 times faster in diamond than it does in the D 2.42 times faster in diamond than it does in the vacuum. vacuum. [This object is a pull tab] https://www.njctl.org/video/?v=LgTHoHC5qtk https://www.njctl.org/video/?v=LgTHoHC5qtk
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