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howto:hambasics:waveinteraction [2020/07/15 21:19] va7fihowto:hambasics:sections:waveinteraction [2021/01/03 08:08] (current) va7fi
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-y|<100% ----- >| 
-|  [[home |Ham Basics]]  |  [[test |About The Test]]  |  [[Reference |References]]  ^  [[sections |Study Sections]]  | 
- 
 ====== Wave Interaction ====== ====== Wave Interaction ======
 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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 {{  refraction_photo_top.png?500  }} {{  refraction_photo_top.png?500  }}
  
-If this last one feels weird to you, imagine this: suppose you're a turtle who can swim twice as fast as 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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   * When going from a "quick" medium to a "slow" medium, light bends away from the surface to spend less time in the slow medium.   * When going from a "quick" medium to a "slow" medium, light bends away from the surface to spend less time in the slow medium.
   * When going from a "slow" medium to a "quick" medium, light does the opposite and bends towards the surface.   * When going from a "slow" medium to a "quick" medium, light does the opposite and bends towards the surface.
 +  * Whatever it does, light always wants to spend less time in a "slow" medium and more time in a "fast" medium because that's the overall quickest way to get from A to B.
  
  
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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:
-  * You can move four points around to see how the refracted ray changes: "//n//<sub>1</sub>", "//n//<sub>2</sub>", "Laser", and "Entry point".+  * You can move four points around to see how the refracted ray changes: "\$n_1\$", "\$n_2\$", "Laser", and "Entry point".
   * 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//<sub>1</sub> and //n//<sub>2</sub> 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.
  
 A few things to try: A few things to try:
-  * Set //n//<sub>1</sub> = 1 and Set //n//<sub>2</sub> = 2 and move the Laser and the Entry Point around.  These are the paths when you can run twice as fast as you can swim.  Notice that if you set //n//<sub>1</sub> = 2 and //n//<sub>2</sub> = 4, or //n//<sub>1</sub> = 2.5 and //n//<sub>2</sub> = 5, it shouldn't matter.  What really matters is the relative speeds between the two media. +  * Set \$n_1 = 1\$ and Set \$n_2 = 2\$ and move the Laser and the Entry Point around.  These are the paths when you can run twice as fast as you can swim.  Notice that if you set \$n_1 = 2\$ and \$n_2 = 4\$, or \$n_1 = 2.5\$ and \$n_2 = 5\$, it shouldn't matter.  What really matters is the relative speeds between the two media. 
-  * Now move the laser in a straight line so that the angle //θ//<sub>1</sub> doesn't change.  The refracted ray shouldn't change either.  So it doesn't matter how far the laser is from the surface.  What matters is the angle at which the beam hits the surface.+  * Now move the laser in a straight line so that the angle \$\theta_1\$ doesn't change.  The refracted ray shouldn't change either.  So it doesn't matter how far the laser is from the surface.  What matters is the angle at which the beam hits the surface.
   * Now move the laser back and forth in a semi circle around the Entry Point.  Although the laser is the same distance away from the Entry Point, the angle of incidence changes so the refracted ray changes.   * Now move the laser back and forth in a semi circle around the Entry Point.  Although the laser is the same distance away from the Entry Point, the angle of incidence changes so the refracted ray changes.
-  * Now set //n//<sub>1</sub> = 1.5 and Set //n//<sub>2</sub> = 1 and play with the laser to change its angle of incidence (<fc #ff0000>important</fc>).  At what angle do you notice that the refracted ray goes parallel to the surface?  This is called the critical angle.  Passed that angle, the ray can't go through and gets reflected instead.+  * Now set \$n_1\$ = 1.5 and Set \$n_2\$ = 1 and play with the laser to change its angle of incidence (<fc #ff0000>important</fc>).  At what angle do you notice that the refracted ray goes parallel to the surface?  This is called the critical angle.  Passed that angle, the ray can't go through and gets reflected instead.
  
  
 ===== Example ===== ===== Example =====
  
-Here's an underwater picture 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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 {{  tirsketch.png  }} {{  tirsketch.png  }}
  
-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  }}
  
  
  
 ===== 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) \$$
  
 {{ refractionreflextion.png?600 }} {{ refractionreflextion.png?600 }}
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 There are four interesting cases here: There are four interesting cases here:
  
-  * If $n_1 < n_2$ (high speed to low speed), then the left hand side of the equation is in danger of being less than the right hand side.  To maintain the equality, $\theta_1 > \theta_2$, which means that the path curves away from the surface. +  * If \$n_1 < n_2\$ (high speed to low speed), then the left hand side of the equation is in danger of being less than the right hand side.  To maintain the equality, \$\theta_1 > \theta_2\$, which means that the path curves away from the surface. 
-  * If $n_1 > n_2$ (low speed to high speed), then the right hand side of the equation is in danger of being less than the left hand side.  To maintain the equality, $\theta_1 < \theta_2$, which means that the path curves away from the surface. +  * If \$n_1 > n_2\$ (low speed to high speed), then the right hand side of the equation is in danger of being less than the left hand side.  To maintain the equality, \$\theta_1 < \theta_2\$, which means that the path curves away from the surface. 
-  * 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>
  
 ====== Scattering ====== ====== Scattering ======
  
-{{: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 }}
  
 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 }}
  
-{{  :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.
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 ====== Diffraction ====== ====== Diffraction ======
  
-{{: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.
  
 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 }}
  
  
 ===== Effect on Communications ===== ===== Effect on Communications =====
  
-{{ :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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 ====== Interference ====== ====== Interference ======
  
-An important property of waves (radio, sound, water, quantum mechanical!, or otherwise) is that they can interfere with one another.  Here's a //Veritasium// video showing how light going through two slits can interfere: In some places, the waves add up, in other places, they cancel out.  Although not directly about radio waves, we saw in the [[intro#electromagnetic_spectrum |intro]] that light and radio waves are in fact on the same electromagnetic spectrum.+An important property of waves (radio, sound, water, quantum mechanical!, or otherwise) is that they can interfere with one another.  Here's a //Veritasium// video showing how light going through two slits can interfere: In some places, the waves add up, in other places, they cancel out.  Although not directly about radio waves, we saw in the [[intro#electromagnetic_spectrum|intro]] that light and radio waves are in fact on the same electromagnetic spectrum.
  
 {{ youtube>Iuv6hY6zsd0 }} {{ youtube>Iuv6hY6zsd0 }}
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 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.
howto/hambasics/sections/waveinteraction.1594873161.txt.gz · Last modified: 2020/07/15 21:19 by va7fi