How does wavelength affect diffraction?

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ranger
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Main Question or Discussion Point

Why does wavelength affect diffraction? For example, red light with a longer wavelength can easily bend around objects. I'm assuming that because of the characteristic longer wavelength, it can just easily bend (flex) as opposed to blue light with would have difficulty becuase of shorter wavelengths. I have this mental picture of red light like a piece of string, where the crests and troughs give the string an apparent wave appearance. Because the crests [and troughs] are further away from each other, the string can easily bend around objects. Is this the right view point?


--thanks.
 

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  • #2
ranger
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No comments on this?
 
  • #3
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Hi,

have a look at this:
http://electron9.phys.utk.edu/phys136d/modules/m9/diff.htm [Broken]

There you can see how the diffraction pattern depends on the wavelength
and why this is so.

I think you shouldn't imagine the light as a string that bends easier around objects if the wavelength is longer.
 
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ranger
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Hi Edgardo,

Thanks for the link. But I fail gather any answer to my question. That article focuses, for the most part, on interference. Am I missing something?
 
  • #5
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Diffraction can be explained by interference, i.e. the superposition (summation) of the Huygens waves. If you calculate for example the diffraction pattern behind the slit you make use of that superposition.

The formula on the website (for the constructive interference) shows you where bright spots will appear behind the slit (maximum). The position of the spots depends on the wavelength.

The greater the wavelength, the greater the angle theta.

But you shouldn't imagine light being some sort of mechanical string that is bent around the opening. Maybe you could elaborate why you think that the "red light string" can bend around corner easier than a blue string.
 
  • #6
ranger
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I didnt know that diffraction and interference where related. I'm thinking that diffraction is the bending of light around an object and interference is where two or more interact, either constructively or destructively. How are these related?

I'm using the string analogy becuase I had no explanation as to how wavelength affects diffraction. If a piece of string [with crests and troughs] has longer wavelengths (red light), I picture it as being least distorted when compared to a straight piece of string. Then assuming that presence of crests and troughs adds stiffness, the red light string would bend easier that the blue light string [becuase less crests and troughs per unit time]. I know this is a weak analogy, but I really couldnt think of anything else.

The formula on the website (for the constructive interference) shows you where bright spots will appear behind the slit (maximum). The position of the spots depends on the wavelength.
Yea I know this. It depends on whether the waves are more or less in/out of phase [and wavelength affects this]. But this doesnt have anything to do with diffraction does it?

thanks again.
 
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  • #7
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Diffraction is the bending of light into the geometrical shadow of an object as you stated correctly. But this is a result of interference.

Let's just concentrate on constructive interference. A maximum appears in the geometrical shadow (for example the first maximum). Thus, by the definition from above this is diffraction.

I recommend the book "Optics" by Eugene Hecht for further information on diffraction and interference.
 

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