Why do EM waves of longer wavelengths spread out more?

In summary: As the slit narrows, the maximum gets wider. In summary, the phenomenon of diffraction explains why longer wavelengths, such as radio waves, spread out more than shorter wavelengths, like optic waves, when encountering an obstacle or gap in a wall. This is due to the process of diffraction, which was first proposed by Huygens and later developed into the modern theory. The amount of spreading depends on the wavelength, with longer wavelengths spreading out faster than shorter wavelengths. This can also be understood through the uncertainty principle, which states that waves cannot be both well-confined in position and momentum. This principle applies to all waves, not just electromagnetic waves. The diffraction pattern can be observed in a ripple tank and is related to the mathematical concept
  • #1
ThunderLight
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Why do longer wavelengths spread out more than shorter wavelengths?
What is the physics principle/law which explains why radio waves spread out more than optic waves in free space?
 
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  • #2
The way that all waves basically behave (EM / sound / ocean etc) when they encounter an obstacle or a gap in a wall, is exactly the same and it is related to wavelength. The process is called Diffraction. Huygens (born 1629) first proposed that you could determine how a wave would progress, thought in terms of 'secondary wavelets', which are constantly forming on the 'wave front' and then add together to form the next step of the wave. Search "Huygens Construction" to see loads of links. In that wiki link, you can see the basics of the process, which later developed into the modern theory of diffraction. The effect scales with wavelength so a wide aperture of object will have the same effect of 'spreading' a wave with long wavelength as a narrow obstruction of similar shape on a proportionally shorter wavelength. Radio waves are just the same as light waves but the wavelength is many millions of times longer.
 
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  • #3
The reason why a beam of visible light spreads out less than a beam of radio waves of the same area can be understood from the uncertainty principle.

The uncertainty principle tells us that waves cannot be both well-confined in position and well-confined in momentum.
The amount that a beam of light spreads depends on its momentum distribution which gives you the distribution of directions of propagation.
With a really short wavelength (i.e., a higher momentum), the same uncertainty in momentum corresponds to a smaller uncertainty in propagation direction.
Therefore, a tightly confined beam will have a larger spread of directions if it is of a longer wavelength.

In this way, one can see that a narrow beam of long wavelength radiation must spread out faster than a short wavelength beam.
 
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  • #4
jfizzix said:
The reason why a beam of visible light spreads out less than a beam of radio waves of the same area can be understood from the uncertainty principle.
Dear old Huygens didn't know about QM when he came up with his Mechanical explanation. Both models have their place, though. HUP is basically an inequality so that makes it difficult to be as precise as Huygens. :smile:
 
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  • #5
ThunderLight said:
Why do longer wavelengths spread out more than shorter wavelengths?
What is the physics principle/law which explains why radio waves spread out more than optic waves in free space?

I'm going to tee off what sophiecentaur has asked, which is have you see examples of wave behavior in a ripple tank? What you are asking is NOT unique to EM wave. It is a property of diffraction in ALL waves.

We can painfully go over Huygens principles, etc., but this particular aspect needs to be clarified so that you know the underlying principle involved here, which is wave mechanics, not just EM wave.

Zz.
 
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  • #6
ThunderLight said:
What is the physics principle/law
It's almost more a matter of Maths than Physics, actually because it is just Patterns following a basic mathematical axiom.
It is strange that the same Maths can apply to so many different phenomena. - right down to two sheep plus two sheep makes four sheep and the same for lollipops and pounds.
All that's necessary for the wave thing to be common is that the propagation medium needs to be uniform and isotropic. When it's not, you get other effects as well.
 
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If you're familiar with the equation for Fraunhofer diffraction from a single slit, the first minimum occurs at an angle θ given by sinθ = λ/w where λ is the wavelength and w is the slit width. The angular width of the central diffraction peak would be 2θ. For a given slit width, the larger the value of λ the wider the central peak will be. But it's best to think in terms of the ratio λ/w.
 
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The thing that confused me was the fact that this Phenomena is usually expressed from the point of view of "Obstacles" and "Slits", while I'm speaking of "Free Space" and "No Obstacles". But, if the Antenna itself that radiates the beam would be considered like that slit, where radiation is produced from it, then I understand.

Thank you all.
Please let me know if my above understanding is not correct.
I will need to go through the related formulas now.
 
  • #9
ThunderLight said:
The thing that confused me was the fact that this Phenomena is usually expressed from the point of view of "Obstacles" and "Slits", while I'm speaking of "Free Space" and "No Obstacles". But, if the Antenna itself that radiates the beam would be considered like that slit, where radiation is produced from it, then I understand.

Thank you all.
Please let me know if my above understanding is not correct.
I will need to go through the related formulas now.
Yes. It's like having a hole in space, letting the EM energy through. That analogy is a bit over-simple because the effective size of that hole is much wider than just the thin metal wire but it holds.
You can think of the diffraction pattern of an infinitely wide slit (i.e. free space) as having a max just in the direction of propagation.
 

1. Why do EM waves of longer wavelengths spread out more?

Electromagnetic (EM) waves with longer wavelengths spread out more because they have lower frequencies. This means that the wave oscillates at a slower rate, causing it to have a longer distance between each wave crest. As a result, the wave appears to spread out more as it travels.

2. Does the medium affect how EM waves spread out?

Yes, the medium through which EM waves travel can affect how much they spread out. In denser mediums, such as water or glass, the waves will spread out less compared to when they travel through less dense mediums, such as air. This is because the density of the medium affects the speed of the wave, which in turn affects the wavelength and frequency.

3. How does the spreading out of EM waves impact their intensity?

As EM waves spread out, their intensity decreases. This is due to the inverse square law, which states that the intensity of a wave is inversely proportional to the square of the distance from the source. Therefore, as the wave spreads out, the distance from the source increases, resulting in a decrease in intensity.

4. Can we manipulate the spreading out of EM waves?

Yes, we can manipulate the spreading out of EM waves through the use of lenses or mirrors. These optical components can focus or diverge the waves, altering their spreading out effect. Additionally, the use of antennas and reflectors can also manipulate EM waves to spread out in specific directions or patterns.

5. Are there any practical applications of EM waves spreading out?

Yes, the spreading out of EM waves has several practical applications. One example is in wireless communication, where antennas are used to transmit and receive signals by manipulating the spreading out of EM waves. Another example is in radio astronomy, where the spreading out of EM waves can provide information about the size and shape of astronomical objects. Additionally, the spreading out of EM waves is essential in medical imaging techniques, such as ultrasound and MRI, which use waves of different wavelengths to produce images of the body.

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