Understanding Wavelength Size in Electromagnetic Waves

[Justin N]

Hey everyone hope you're doing well.
I've always been confused about one particular aspect of electromagnetic waves. I understand that each frequency has a corresponding wavelength that is inversely proportionate and that the wavelength is the distance between successive crests or troughs. But does the actual size of the wave change (height?) or just the distance between successions?
Sorry for the length of post and thanks for any info.

 

[mishima]

In short, the distance between successions.

Imagine a human wave at a baseball game. There, its the humans height changing over time.

But in a real EM wave, its not humans but the value of the electromagnetic field. You can think of it as an arrow growing and shrinking, but its really just a number (force per charge) attached to a certain point in space.

There's no instrument that can directly measure that value (closest is voltage maybe) but consider analogy of a digital thermometer stuck in a pig in the oven. The number associated with the tip of the probe would change over time. Same thing happens in space with an EM field when radiation propagates through.

Wavelength is the distance between points with the same value in a repeating pattern during one cycle of whatever is causing the wave. Typically, this is one oscillation of an electron.

 

[Justin N]

Thank you for sorting that out for me, I appreciate it.
Out of curiosity, what exactly is electron oscillation. I'm going to have to research that one. Thanks again.

 

[Drakkith]

Out of curiosity, what exactly is electron oscillation.

One example would be the alternating current created in a antenna for a radio transmitter. The electrons oscillate back and forth at the same frequency as the transmitting signal. This sets up an alternating electric and magnetic disturbance in the EM field that propagates outwards as an EM wave.

 

[mishima]

Check out this little simulation. It shows a positive charge (for some reason) and the electric field lines. You are able to move the charge around. Watch what happens to the field when you do (you get a wave).

When you move the charge back and forth in a regular way (which is what oscillation means) you get a wave with a frequency that matches how fast you oscillate it.

https://phet.colorado.edu/sims/radiating-charge/radiating-charge_en.html

 

[Justin N]

Check out this little simulation. It shows a positive charge (for some reason) and the electric field lines. You are able to move the charge around. Watch what happens to the field when you do (you get a wave).

When you move the charge back and forth in a regular way (which is what oscillation means) you get a wave with a frequency that matches how fast you oscillate it.

https://phet.colorado.edu/sims/radiating-charge/radiating-charge_en.html

Going to have to watch the video when I have access to my pc but thanks for the quick and informative reply. That all makes a lot more sense now. I appreciate your time.

 

[Justin N]

One example would be the alternating current created in a antenna for a radio transmitter. The electrons oscillate back and forth at the same frequency as the transmitting signal. This sets up an alternating electric and magnetic disturbance in the EM field that propagates outwards as an EM wave.

Thanks for the reply. The antenna example was very helpful and brought it closer to home.

 

[Justin N]

https://phet.colorado.edu/sims/radiating-charge/radiating-charge_en.html

That simulation is pretty trippy to look at but it definitely serves its purpose.
So I'm assuming the fields radiate into all three dimensions with equal strength?
Sorry for the follow up questions.

 

[mishima]

Right, but in reality it depends greatly on the antenna design. An omnidirectional antenna goes in all directions, but others not so much. Here's a power pattern (showing its "strength" in all directions) for a more directional transmitter:

 

[davenn]

So I'm assuming the fields radiate into all three dimensions with equal strength?

for a single charge in free space, yes

for charges in an antenna, no. No antenna radiates equally in all directions
the closest you can get is with a dipole antenna which will produce a doughnut pattern
there will be near zero radiation off the ends of the dipole

side on and above ...

Dave

 

[Justin N]

View attachment 209453

Interesting. Well thanks for all of the info Mishima and for all of the visuals, they really helped me imagine what's going on.

 

[Justin N]

for a single charge in free space, yes

for charges in an antenna, no. No antenna radiates equally in all directions
the closest you can get is with a dipole antenna which will produce a doughnut pattern
there will be near zero radiation off the ends of the dipole

side on and above ...
View attachment 209464

View attachment 209465Dave

Thanks for all the help Davenn. The visual really helps put it together.

 

[Drakkith]

Thanks for all the help Davenn. The visual really helps put it together.

Just to be clear, are you aware that those illustrations represent the intensity of the EM wave generated by the antenna and not the shape of the wave?

 

[Justin N]

Just to be clear, are you aware that those illustrations represent the intensity of the EM wave generated by the antenna and not the shape of the wave?

Negative, I did not know that. You've thrown me back into the abyss Drakkith. Haha I'm going to have to think about this one again

 

[FactChecker]

But does the actual size of the wave change (height?) or just the distance between successions?

The height of the wave is a way to graphically visualize the energy (or power) in the wave, not the frequency. Waves of the same frequency but different energy would be graphically represented with the same distance from peak to peak but the height of the peaks would be different. You can also use the position of the peaks to represent the phase that the wave is in. If two waves are mixed together, you can add their heights to see how they interact.

 

[Justin N]

The height of the wave is a way to graphically visualize the energy (or power) in the wave, not the frequency. Waves of the same frequency but different energy would be graphically represented with the same distance from peak to peak but the height of the peaks would be different. You can also use the position of the peaks to represent the phase that the wave is in. If two waves are mixed together, you can add their heights to see how they interact.

Ok, that's what I had thought. But on some electromagnetic spectrum charts it shows the size of the wave being as large as a mountain or a human or a cell, etc. So that's where my confusion comes from in terms of my original post. Now I understand they were talking about the distance between successive points on a wave and not the actual "height".

 

[davenn]

Just to be clear, are you aware that those illustrations represent the intensity of the EM wave generated by the antenna and not the shape of the wave?

Negative, I did not know that. You've

just to be even more clearer... you were asking about an antenna/charge radiating evenly in ALL directions
my illustrations are showing you that for, eg., a dipole antenna they don't ... which is what I stated

you can clearly see that the intensity of the signal varies depending on where it is measured relative to the antenna
Strongest directly perpendicular to the sides and near zero off the ends of the dipole

The illustrations are showing you intensity Vs direction from the antenna

does that make sense to you ?

Dave

 

[Justin N]

you can clearly see that the intensity of the signal varies depending on where it is measured relative to the antenna
Strongest directly perpendicular to the sides and near zero off the ends of the dipole
The illustrations are showing you intensity Vs direction from the antenna
does that make sense to you ?

Dave

I believe so. I understand the strength of intensity shown by the color. I need to do some research on a dipole antenna though. Thanks for the info.