# EM Waves (waves or pulses)?

1. Oct 20, 2014

### mikewday

I am not looking for formulas or references to theories... this is just a general question about how to visualize/explain EM waves.

Are EM waves such as light, radio waves, etc. really more like pulses and we just measure their intensity from a static point as a wave line? I thought I remembered reading something like that somewhere and it seemed to make sense when I tried to think about light "waves" travelling in all directions at once.

2. Oct 20, 2014

### Staff: Mentor

You can visualize them either way. If you just put a single detector at a single point while an electromagnetic wave is flowing by, that dectector will record a positive electrical field, falling to zero, going negative, and then swinging back to positive again as the cycle repeats: $E=sin\omega{t}$ where $t$ is the time. You can describe that behavior as the electrical field "pulsing" at that one point. However, if you look also measure the electrical field strength at another nearby point, the waviness will become apparent: the highs and lows there will happen sooner or later than at the first point by a fixed amount and you'll see that you're looking at the peaks and troughs of an electromagnetic wave passing through.

3. Oct 20, 2014

### sophiecentaur

I don't think you can expect to get very far along those lines - except for the very simplest idea of wave propagation,in general. This is based on the notion that a disturbance at some point in a medium takes some time to make itself felt at a distance. That introduces a delay, which implies a finite propagation time or speed for the disturbance.
Have you looked at Wikipedia or anywhere else for some already prepared descriptions? The descriptions of wave propagation usually tend to involve a spot of Maths because it is such a good way of describing what goes on (the way the shape of the wave varies with both distance and time, for instance).

4. Oct 24, 2014

### mikewday

The reason I am asking is that I want to be able to explain EM waves to young kids in a way that makes logical sense using things they can experience and understand. I think it is important for younger kids to picture some of the concepts before diving into the math. You always see EM waves represented as wavy lines but, isn't that just a mathmatical representation of a measurement at a certain location over a period of time? If not, how else could I think about it ina touch-ie feel-ie way?

If I think of it like the following, conceptually at least, does it makes sense? I need a way that we can relate to them with physical things we can work with.
If you were to perform some action (ie: click a clicker) in the center of a pool of water (gravity aside), similar presure waves would propagate out in all directions at once. If I were to measure those presure differences as they passed by in mutiple locations in that pool of water, I would get a wave pattern on my measurement tool. In water, I would think that wave line would get longer the further out from the center that the measurement device is placed. EM waves, in contrast, would have the same wavelength no matter how far away they were measured correct? (Unless they interacted with something along the way). To me, that is the interesting part about EM waves... I don't have a great understanding of the math myself but, it seems we can only use math to describe them because we do not have anything similar that we can touch/feel with our senses that acts the same way. We can only get close to explaining it with things we can see and touch. A presure wave in water is about the closest I can think as a way to show an example using touchable things. I am open to any ideas...

5. Oct 24, 2014

### sophiecentaur

My Dad gave me an excellent explanation. He talked about taking a bar magnet and suddenly reversing the way it points. A small compass, at a distance, will change direction to follow the magnet - but there will be a delay. If you keep reversing the magnet (rotating it, even) it will produce a set of waves that spread out, each peak and trough corresponding to the orientation of the magnet some time previously. and the compass will keep following what the magnet does (did). The analogy with a transmitter and a receiver is fairly easy to grasp, I think and what is actually happening is the generation of a radiating electromagnetic wave.

6. Oct 24, 2014

### Staff: Mentor

It's also a mathematical representation of a set of measurements at different locations at the same time, a sort of snapshot. The complete description is in effect a series of these snapshots at successive times, revealing how the pattern of measurements travels as time passes.

Also note that the concept of "wave" is not limited to the sinusoidal waves that we often make pictures of and analyze mathematically using sine and cosine functions. These are simply common forms of waves that use common mathematical functions for their description. If you shake one end of a rope up and down once, quickly, and produce a single "pulse" that travels along the rope, that is just as much a wave as the sinusoidal kind. The essence of a wave is that you have a "pattern of disturbance" that propagates through a medium (rope, water surface, electromagnetic field, etc.).

7. Oct 24, 2014

### DevendraC

You have to first explain about the wave nature of light. As we know that light is a beam of photons and photons act as particles in some cases (Photoelectric effect) and as waves(Young's Experiment) in some cases. Yeah, that is dual nature of light. So, if you take Young's diffraction experiment you get alternate dark and bright bands on the screen. Now, relate these alternate bright and dark bands with your sinusoidal wave. The bright bands represent the crests where as the dark bands represent the troughs and the interesting thing is your equation does have a sine function in it. So, there is a relation between the practical experiments and the theoretical paper work. Lastly, according to me, if you explain the children about the wave nature of light by taking simple examples like Young's diffraction experiment, as i have stated earlier, and relate the outputs of those experiments with given mathematical equations, it would be too easier to understand the topic for the children.

8. Oct 24, 2014

### mikewday

Thanks for all of the information. :-)

When I say kids... I mean 10 or 11 year olds. I am trying to keep a complex topic as simple as possible but still have some validity. I think kids need a basic grounding in how to visualize some of these concepts before they start diving into a lot of details.

I like to explain a topic like heat to my daughter by saying it is basically motion at a molecular level... it's how fast molecules are bouncing around. The faster they are moving, the futher they bounce apart and the more space they take up.

9. Oct 25, 2014

### sophiecentaur

Heat ('internal energy') is just how you say it is and the basics of Kinetic Theory are straightforward for 10/11 year olds. It's basically an extension of concrete 'mechanical' stuff and I'm sure you have found the ideas readily accepted. Heat transfer by Radiation is a massive step and can be misinterpreted so easily.
Waves are a more sophisticated matter because you can't 'see' EM waves (not the details of the shape). The phenomenon of Interference is really hard to grasp without some of the ideas involved in less elementary Maths - so you can't even reason your way into justifying light being a wave, although you can use the common Long Wave, Medium Wave descriptions for radio frequency waves because of familiarity with those terms. It's all so very abstract, though. It isn't helped by the Photon model, which is usually presented to kids far too early and in terns of the 'little bullets' that went with the Corpuscular Theory of Light. Jumping into Quantum Theory is , imo, totally inappropriate for such young and inexperienced minds but National Curricula seem to insist that light particles should be taught (the blind leading the blind, as far as I can see, in lower school teaching).

10. Oct 25, 2014

### Staff: Mentor

I think that you need to decide what knowledge you want your audience to come away with. "Visualize/explain EM waves" is incredibly vague, you set yourself up for failure automatically by having such a vague goal.

If I were working with 10-11 year olds I would first recognize that they are still predominantly concrete thinkers, or just barely moving from concrete to abstract thought. Understanding the mechanisms of EM waves is likely to be too abstract. My goal would be to have them learn about the wave behavior of light (diffraction, refraction, interference, etc.).

You can take examples of water, string, and sound waves to illustrate the same concepts for EM. This is a good way to help move from concrete thought about specific kinds of waves to abstract thoughts about characteristics that are common to waves of all types. The mechanism of the various waves is less important than their common behaviors.

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