One question about diffraction

In summary: The first criterion is the object's size- if it's smaller than the wavelength of the radiation then it will just pass around it. If it's bigger, but the size of the wave is less than the diameter of the object, then part of the wave will reflect off of it and be detected. The second criterion is the polarization of the radiation- if the radiation is polarized in a certain way then the object will scatter more of the radiation than if it's unpolarized. The third criterion is the orientation of the object- if the object is oriented in a particular way then parts of the wave will be reflected in a specific direction.
  • #1
Eagle9
238
10
When the electromagnetic wave propagates in air and it meets the object with the size less than the wavelength then this wave simply pass around this object and continues its way as if this object does not exist at all.
Now imagine such situation. The antenna emit the radio waves with the wavelength of let’s say 1 meter and it meet the metallic cylinder with the height/length of 100 meters but width/diameter of 0.1 meter. What will happen? One geometric parameter (cylinder’s length) is more than wavelength, but the second (width) is less, will the radio wave pass around this cylinder or will it reflect from cylinder? :rolleyes:
Difraction.png
 
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  • #2
The effect of this cylinder on a passing wave will depend upon the polarisation. If the E field is at right angles to the axis of the cylinder, there will be very little effect but if the polarisation (E Field) is parallel with the wave the scattering can be significant. The amount of the scattered power (and, hence the effect on the incident wave front) will depend on the actual length of the cylinder in wavelengths.
A conductor of any size will have some effect on an incident wave so there will always some degree of diffraction - certainly a half wavelength wire behaves like a plate with cross section of about 1/2 by 1/2 wavelengths when the polarisation is parallel to the wire.
 
  • #3
"When the electromagnetic wave propagates in air and it meets the object with the size less than the wavelength then this wave simply pass around this object and continues its way as if this object does not exist at all."

This is not true. The wave would scatter.
 
  • #4
Eagle9 said:
When the electromagnetic wave propagates in air and it meets the object with the size less than the wavelength then this wave simply pass around this object and continues its way as if this object does not exist at all.

Where did you get this idea? It's clearly false- the subwavelength object will scatter the incident radiation, well described by Rayleigh scattering (The mechanism for the sky appearing blue).
 
  • #5
Andy Resnick said:
Where did you get this idea? It's clearly false- the subwavelength object will scatter the incident radiation, well described by Rayleigh scattering (The mechanism for the sky appearing blue).

It's the sort of thing that they get told at School as a throw-away remark, I expect.
 
  • #6
When the size of object is comparable to wavelength ,then one must take into account the interaction.However if the object is of much bigger size,then ray optics will do the work(it is worth noting that there are three criterias)
 
  • #7
It's hard to say when scattering due to small 'obstacles' may or may not be a problem. It's situation specific.
 
  • #8
Meir Achuz
Andy Resnick
sophiecentaur
I learned Physics in school only and not in the university, besides English is not my native language, so perhaps I made a mistake when writing this:
When the electromagnetic wave propagates in air and it meets the object with the size less than the wavelength then this wave simply pass around this object and continues its way as if this object does not exist at all.
:smile:

Now:
The effect of this cylinder on a passing wave will depend upon the polarisation. If the E field is at right angles to the axis of the cylinder, there will be very little effect but if the polarisation (E Field) is parallel with the wave the scattering can be significant. The amount of the scattered power (and, hence the effect on the incident wave front) will depend on the actual length of the cylinder in wavelengths.
So, if the E field is parallel to cylinder then this cylinder will be able to scatter that wave and hence some detector (radio receiver or something like this) will be able to “see” the object/cylinder, right? Did I understand everything correctly? :rolleyes:

A conductor of any size will have some effect on an incident wave so there will always some degree of diffraction - certainly a half wavelength wire behaves like a plate with cross section of about 1/2 by 1/2 wavelengths when the polarisation is parallel to the wire.
But what happens if we have got the cylinder made of plastics? This substance is insulator, not conductor :smile:

Andy Resnick
Where did you get this idea? It's clearly false- the subwavelength object will scatter the incident radiation, well described by Rayleigh scattering (The mechanism for the sky appearing blue).
Well, than tell me-why the optical microscopes cannot see the objects with the size less than wavelength of the light? Why do we need to use the electric microscopes?

andrien
When the size of object is comparable to wavelength ,then one must take into account the interaction.However if the object is of much bigger size,then ray optics will do the work(it is worth noting that there are three criterias)
May I know which ones?
 
  • #9
Eagle9 said:
May I know which ones?

The three criterias are simply for the fields,the zones in which one is important.First is if the object size is <λ,second is between λ and object size and third you know which one.
 
  • #10
Eagle9 said:
<snip>
Well, than tell me-why the optical microscopes cannot see the objects with the size less than wavelength of the light? Why do we need to use the electric microscopes?

Your questions are unrelated. One question concerns wave propagation in an inhomogeneous medium, while the other concerns imaging. Consider- fluorescent molecules are much smaller than the wavelength of light they emit.

Additionally, there are imaging methods that can image (as opposed to detect) sub-wavelength sized objects: near-field scanning, for example. Other techniques can localize single fluorescent molecules to volumes much less than the Abbe limit: STORM, for example.
 

1. What is diffraction?

Diffraction is a phenomenon that occurs when a wave, such as light or sound, encounters an obstacle or slit that is comparable in size to its wavelength. This causes the wave to bend and spread out, creating a pattern of interference.

2. How does diffraction affect our daily lives?

Diffraction plays a role in many everyday activities, such as hearing, seeing, and using technology. It allows us to hear sound from around corners, see colors in soap bubbles and oil slicks, and use diffraction gratings in devices like CD players and spectacles.

3. What are some examples of diffraction in nature?

Diffraction is commonly seen in nature, such as in the iridescent colors of butterfly wings and peacock feathers. It is also responsible for the colorful patterns seen in soap bubbles, the shimmering colors of oil slicks, and the rainbow-like patterns of light passing through a prism.

4. How is diffraction different from refraction and reflection?

Diffraction, refraction, and reflection are all properties of waves, but they differ in how they interact with an obstacle or boundary. Diffraction occurs when a wave bends around an obstacle or opening, refraction occurs when a wave changes direction as it passes through a medium, and reflection occurs when a wave bounces off a surface.

5. How is diffraction used in scientific research?

Diffraction is used in various fields of science, such as physics, chemistry, and biology, to study the structure and behavior of matter. For example, X-ray diffraction is commonly used to study the atomic and molecular structure of crystals, while electron diffraction is used to study the structure of molecules and materials at the nanoscale.

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