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| howto:hambasics:sections:waveinteraction [2020/10/07 08:08] – va7fi | howto:hambasics:sections:waveinteraction [2021/01/03 08:08] (current) – va7fi |
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| When an electromagnetic wave (radio, light, etc) hits a surface, it can do one or a mix of six things((Picture from [[http://www.mrwaynesclass.com/lightOptics/reading/index02.html]])): | When an electromagnetic wave (radio, light, etc) hits a surface, it can do one or a mix of six things((Picture from [[http://www.mrwaynesclass.com/lightOptics/reading/index02.html]])): |
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| {{ ..:lightbehavior.png?500 }} | {{ lightbehavior.png?500 }} |
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| Let's start with refraction and reflection. | Let's start with refraction and reflection. |
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| **Option 1**: You make a B-line for the child because the shortest distance between two points is a straight line. | **Option 1**: You make a B-line for the child because the shortest distance between two points is a straight line. |
| {{ ..:beach1.png }} | {{ beach1.png }} |
| While it's true that this straight line is the //shortest// distance, it's not necessarily the //fastest// path. The problem here is that the water slows you down too much. It's better to cover more ground where you're faster and less where you're slower. | While it's true that this straight line is the //shortest// distance, it's not necessarily the //fastest// path. The problem here is that the water slows you down too much. It's better to cover more ground where you're faster and less where you're slower. |
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| **Option 2**: You run until you're as close to the child as possible before jumping in the water to swim as little as possible. | **Option 2**: You run until you're as close to the child as possible before jumping in the water to swim as little as possible. |
| {{ ..:beach2.png }} | {{ beach2.png }} |
| That path might be faster than the previous one, but it's not the fastest. Here's a thought experiment: | That path might be faster than the previous one, but it's not the fastest. Here's a thought experiment: |
| * Imagine that you could swim as fast as you can run, then Option 1 would be the fastest path because there would be no difference between running or swimming so the shortest path would also be the quickest. | * Imagine that you could swim as fast as you can run, then Option 1 would be the fastest path because there would be no difference between running or swimming so the shortest path would also be the quickest. |
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| **Option 3**: For regular running speeds, the quickest path is to enter the water somewhere in between. | **Option 3**: For regular running speeds, the quickest path is to enter the water somewhere in between. |
| {{ ..:beach3.png }} | {{ beach3.png }} |
| It turns out that, people have a pretty good intuition of where that "somewhere" is. But using Calculus, it's possible to find exactly where to enter the water to get to the child as quickly as possible. | It turns out that, people have a pretty good intuition of where that "somewhere" is. But using Calculus, it's possible to find exactly where to enter the water to get to the child as quickly as possible. |
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| If you shine a beam of light through a piece of glass, it will bend so as to get to the other side as quickly as possible.((Picture modified from [[wp>Refractive_index |Wikipedia: Refractive Index]])) | If you shine a beam of light through a piece of glass, it will bend so as to get to the other side as quickly as possible.((Picture modified from [[wp>Refractive_index |Wikipedia: Refractive Index]])) |
| {{ ..:refraction_photo_bottom.png?500 }} | {{ refraction_photo_bottom.png?500 }} |
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| This principle is called [[wp>Fermat's_principle |Fermat's Principle of Least Time]] and in first year Calculus, students use this principle to derive [[wp>Snell's_law |Snell's Law of Refraction]], taught in high school physics, which relates the angles of incidence and refraction to the [[wp>Refractive_index |refractive indices]]. | This principle is called [[wp>Fermat's_principle |Fermat's Principle of Least Time]] and in first year Calculus, students use this principle to derive [[wp>Snell's_law |Snell's Law of Refraction]], taught in high school physics, which relates the angles of incidence and refraction to the [[wp>Refractive_index |refractive indices]]. |
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| But what if light goes into a medium where it can travel faster? Then this happens: | But what if light goes into a medium where it can travel faster? Then this happens: |
| {{ ..:refraction_photo_top.png?500 }} | {{ refraction_photo_top.png?500 }} |
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| If this last one feels weird to you, imagine this: suppose you're a turtle who can swim faster than you can walk. It makes sense that you'd want to spend more time in the water and less on the beach: | If this last one feels weird to you, imagine this: suppose you're a turtle who can swim faster than you can walk. It makes sense that you'd want to spend more time in the water and less on the beach: |
| {{ ..:beach4.png }} | {{ beach4.png }} |
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| To recap: | To recap: |
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| This second case (going from a "slow" medium to a "fast" medium) is really interesting because at some point, the light beam bends so much that it "exits" parallel to the surface, and then reflects like a mirror:((Picture from [[wp>Total_internal_reflection |Wikipedia: Total Internal Reflection]])) | This second case (going from a "slow" medium to a "fast" medium) is really interesting because at some point, the light beam bends so much that it "exits" parallel to the surface, and then reflects like a mirror:((Picture from [[wp>Total_internal_reflection |Wikipedia: Total Internal Reflection]])) |
| {{ ..:refractionreflextion.png?600 }} | {{ refractionreflextion.png?600 }} |
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| This behaviour is a bit hard to explain without going into the math, but here's an animation that allows you to explore it: | This behaviour is a bit hard to explain without going into the math, but here's an animation that allows you to explore it: |
| * Note though that this particular animation only works if the laser is below the horizontal line. | * Note though that this particular animation only works if the laser is below the horizontal line. |
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| \$n_1\$ and \$n_2\$ are the [[wp>Refractive_index |Refractive Indices]] of the media. They are defined as the ratio of the speed of light in vacuum to the speed of light in the media \$\left(n = \frac{c}{v}\right)\$. For example, if \$n = 2\$, then the speed of light is twice as //slow// in the medium as it is in vacuum. The bigger \$n\$ is, the slower the speed. \$n = 1\$ means that the speed is the same as the speed of light in a vacuum. | \$n_1\$ and \$n_2\$ are the [[wp>Refractive_index |Refractive Indices]] of the media. They are defined as the ratio of the speed of light in vacuum to the speed of light in the media \$\left(n = \frac{c}{v}\right)\$. For example, if \$n = 2\$, then the speed of light is twice as //slow// in the medium as it is in vacuum. The bigger \$n\$ is, the slower the speed. \$n = 1\$ means that the speed is the same as the speed of light in a vacuum. |
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| Here's an underwater picture that VA7FI took in a lake with a waterproof camera. The camera is completely submerged under water looking up toward the surface. Above a certain angle, it's possible to see the beach, trees, and the sky. But below that angle, we see the reflection of his wetsuit. | Here's an underwater picture that VA7FI took in a lake with a waterproof camera. The camera is completely submerged under water looking up toward the surface. Above a certain angle, it's possible to see the beach, trees, and the sky. But below that angle, we see the reflection of his wetsuit. |
| {{ ..:laketir.jpg }} | {{ laketir.jpg }} |
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| Here's a sketch of the setup: | Here's a sketch of the setup: |
| {{ ..:tirsketch.png }} | {{ tirsketch.png }} |
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| The other cool thing about that picture is that if you zoom in on the beach, you'll see the colours separate (as if through a prism). This indicates that the index of refraction, //n//, depends on the frequency. This will be important when we relate all of this back to radio waves. | The other cool thing about that picture is that if you zoom in on the beach, you'll see the colours separate (as if through a prism). This indicates that the index of refraction, //n//, depends on the frequency. This will be important when we relate all of this back to radio waves. |
| {{ ..:laketirzoom.jpg?650 }} | {{ laketirzoom.jpg?650 }} |
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| ===== Snell's Law (Optional) ===== | ===== Snell's Law (Optional) ===== |
| | <hidden> |
| Snell's law gives the relationship between the angle of incidence and refraction depending on the refraction indices: | Snell's law gives the relationship between the angle of incidence and refraction depending on the refraction indices: |
| \$$ n_1 \sin(\theta_1) = n_2 \sin(\theta_2) \$$ | \$$ n_1 \sin(\theta_1) = n_2 \sin(\theta_2) \$$ |
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| {{ ..:refractionreflextion.png?600 }} | {{ refractionreflextion.png?600 }} |
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| * If we keep increasing \$n_1\$ compared to \$n_2\$, then \$\theta_2\$ can increase to the point where it's going parallel to the surface (\$\theta_2 = 90^\circ\$), which means that: \$\frac{n_1}{n_2} \sin(\theta_1) = 1 \$. At this point, we call \$\theta_1\$ the critical angle. | * If we keep increasing \$n_1\$ compared to \$n_2\$, then \$\theta_2\$ can increase to the point where it's going parallel to the surface (\$\theta_2 = 90^\circ\$), which means that: \$\frac{n_1}{n_2} \sin(\theta_1) = 1 \$. At this point, we call \$\theta_1\$ the critical angle. |
| * If we keep increasing \$n_1\$ even further, then \$\frac{n_1}{n_2} \sin(\theta_1) >1 \$, which means that it's impossible for \$\theta_2\$ to keep up since \$\sin(\theta_2) \leq 1\$. This is when Total Internal Reflection occurs, which is what we use to "bounce" radio waves off the ionosphere (more on that next). | * If we keep increasing \$n_1\$ even further, then \$\frac{n_1}{n_2} \sin(\theta_1) >1 \$, which means that it's impossible for \$\theta_2\$ to keep up since \$\sin(\theta_2) \leq 1\$. This is when Total Internal Reflection occurs, which is what we use to "bounce" radio waves off the ionosphere (more on that next). |
| | </hidden> |
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| ====== Scattering ====== | ====== Scattering ====== |
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| {{:howto:hambasics:scattering.png?85 }} | {{howto:hambasics:sections:scattering.png?85 }} |
| Scattering occurs when an EM wave hits a bunch of "small particles" that in turn re-radiate the wave in all direction. Note that the "small particles" can be single atoms, molecules, dust, or pockets of gas with a different index of refraction. They can also be bigger objects like meteors or small planes! The size of the "particle" is always relative to the wavelength of the EM wave. To a 160m radio wave, a meteor is small, but to a laser beam (≈500nm), a dust particle is very big. | Scattering occurs when an EM wave hits a bunch of "small particles" that in turn re-radiate the wave in all direction. Note that the "small particles" can be single atoms, molecules, dust, or pockets of gas with a different index of refraction. They can also be bigger objects like meteors or small planes! The size of the "particle" is always relative to the wavelength of the EM wave. To a 160m radio wave, a meteor is small, but to a laser beam (≈500nm), a dust particle is very big. |
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| The first picture shows a laser beam shinning at the wall. | The first picture shows a laser beam shinning at the wall. |
| {{ :howto:hambasics:laser1.jpg }} | {{ howto:hambasics:sections:laser1.jpg }} |
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| In the second picture, water is sprayed into the path of the laser beam. | In the second picture, water is sprayed into the path of the laser beam. |
| {{ :howto:hambasics:laser2.jpg }} | {{ howto:hambasics:sections:laser2.jpg }} |
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| {{ :howto:hambasics:lightscattering.jpg}}The reason the beam is invisible in the first picture is that all the light from the laser travels toward the wall (and none toward the camera). But in the second picture, the water vapour scatters some of that light in random directions, allowing some of it to reach the camera. There's a subtle point here: light from a regular light bulb also does this. What I mean is this: | {{ howto:hambasics:sections:lightscattering.jpg}}The reason the beam is invisible in the first picture is that all the light from the laser travels toward the wall (and none toward the camera). But in the second picture, the water vapour scatters some of that light in random directions, allowing some of it to reach the camera. There's a subtle point here: light from a regular light bulb also does this. What I mean is this: |
| * Look at the light bulb in the room you're in. | * Look at the light bulb in the room you're in. |
| * Now look at an object that the light bulb illuminates. | * Now look at an object that the light bulb illuminates. |
| ====== Diffraction ====== | ====== Diffraction ====== |
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| {{:howto:hambasics:diffraction.png }} | {{howto:hambasics:sections:diffraction.png }} |
| Diffraction is the bending of waves around the corners of an obstacle or through an aperture. The diffracting object or aperture effectively becomes a secondary source of the propagating wave, which in turns can interact with the main wave or other diffracted waves. | Diffraction is the bending of waves around the corners of an obstacle or through an aperture. The diffracting object or aperture effectively becomes a secondary source of the propagating wave, which in turns can interact with the main wave or other diffracted waves. |
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| All waves do this to an extent, but the phenomena is most pronounced when the the wavelength is of the same order as the size of the diffracting object.((The two pictures of diffraction were adapted from [[http://www.saburchill.com/physics/chapters2/0008.html]])) | All waves do this to an extent, but the phenomena is most pronounced when the the wavelength is of the same order as the size of the diffracting object.((The two pictures of diffraction were adapted from [[http://www.saburchill.com/physics/chapters2/0008.html]])) |
| {{ :howto:hambasics:diffraction_01.jpg }} | {{ howto:hambasics:sections:diffraction_01.jpg }} |
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| ===== Effect on Communications ===== | ===== Effect on Communications ===== |
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| {{ :howto:hambasics:wave-diffraction-radio.gif}} | {{ howto:hambasics:sections:wave-diffraction-radio.gif}} |
| Since radio waves can bend around obstacles that are similar in size to the wavelength of the signal, lower frequencies can band over hills and travel beyond the horizon as ground waves because of diffraction (more on this later).((Image of the radio tower and mountain is from [[https://kistodaynews.com/2017/09/12/scitech-magazine-waves/]])) | Since radio waves can bend around obstacles that are similar in size to the wavelength of the signal, lower frequencies can band over hills and travel beyond the horizon as ground waves because of diffraction (more on this later).((Image of the radio tower and mountain is from [[https://kistodaynews.com/2017/09/12/scitech-magazine-waves/]])) |
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| Here's a computer animation from [[wp>Wave_interference#Mechanisms |Wikipedia]] showing the same principle: | Here's a computer animation from [[wp>Wave_interference#Mechanisms |Wikipedia]] showing the same principle: |
| {{ ..:two_sources_interference.gif }} | {{ two_sources_interference.gif }} |
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| In terms of radio signals, every time you have more than one source (either because of reflection or because of another radio or antenna), you'll have regions where the signal fades and regions where it increases. Here's why... | In terms of radio signals, every time you have more than one source (either because of reflection or because of another radio or antenna), you'll have regions where the signal fades and regions where it increases. Here's why... |
| The first thing to notice is that when a wave reflects off a surface, it suffers a half-wavelength phase shift. This means that if the receiver is right next to the "mirror", the signal will cancel out. | The first thing to notice is that when a wave reflects off a surface, it suffers a half-wavelength phase shift. This means that if the receiver is right next to the "mirror", the signal will cancel out. |
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| If the receiver then moves away from the "mirror", the <fc #008000>reflected signal</fc> has to travel over a longer distance than the <fc #4682b4>direct signal</fc> before reaching the receiver. This means that phase between the two waves will change, sometimes cancelling each other, sometimes reinforcing each other. When the path difference (Δ) between the reflected and direct waves is a whole number of the wave length, the two waves cancel each other because of the half-wavelength difference from the reflection. But when the difference is a multiple of a half wavelength, the two waves add up constructively and the resulting signal is stronger. | If the receiver then moves away from the "mirror", the <fc #008000>reflected signal</fc> has to travel over a longer distance than the <fc #4682b4>direct signal</fc> before reaching the receiver. This means that phase between the two waves will change, sometimes cancelling each other, sometimes reinforcing each other. When the path difference (Δ) between the reflected and direct waves is a whole number of the wave length, the two waves cancel each other because of the half-wavelength difference from the reflection. But when the difference is a multiple of a half wavelength, the two waves add up constructively and the resulting signal is stronger. |
