| Next revision | Previous revision |
| hambasics:sections:waveinteraction [2021/01/03 08:08] – created - external edit 127.0.0.1 | hambasics:sections:waveinteraction [2024/11/24 12:57] (current) – va7fi |
|---|
| * 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. |
| |
| {{ggb>/howto/hambasics/sections/snells2.ggb 705,403}} | {{ggb>/hambasics/sections/snells2.ggb 705,403}} |
| |
| \$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. |
| ====== Scattering ====== | ====== Scattering ====== |
| |
| {{howto:hambasics:sections:scattering.png?85 }} | {{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. |
| |
| |
| The first picture shows a laser beam shinning at the wall. | The first picture shows a laser beam shinning at the wall. |
| {{ howto:hambasics:sections:laser1.jpg }} | {{ 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:sections:laser2.jpg }} | {{ hambasics:sections:laser2.jpg }} |
| |
| {{ 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: | {{ 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 ====== |
| |
| {{howto:hambasics:sections:diffraction.png }} | {{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:sections:diffraction_01.jpg }} | {{ hambasics:sections:diffraction_01.jpg }} |
| |
| |
| ===== Effect on Communications ===== | ===== Effect on Communications ===== |
| |
| {{ howto:hambasics:sections:wave-diffraction-radio.gif}} | {{ 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/]])) |
| |
| 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. |
| |
| {{ggb>/howto/hambasics/sections/multipath.ggb 700,300}} | {{ggb>/hambasics/sections/multipath.ggb 700,300}} |
| |
| 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. |
