As a complete layman/ignorant…could anyone breakdown what EXACTLY are radio-waves,what physical properties do they have, they have a weight and so on??
And where do you fit the phenomenon of “light” into this, Im aware that light have different wavelength and so on, but what exactly IS IT?, light/radio-waves are different things on the same spectrum?
This has been intrigued be for quite some time. I really need this explained as you would to a 5-year old!
appreciate any pedagogic answer
A very good question. The answer is one of the triumphs of 19th century physics, the theory of electromagnetism, developed by James Clerk Maxwell.
Maxwell combined all the known facts about electricity and magnetism and developed a general theory, which predicted many things. A magnetic field causes magnets to move. An electric field causes electrically charged particles like electrons to move. A changing electric field gives rise to a changing magnetic field. In turn, a changing magnetic field creates an electric field.
Maxwell wrote out a mathematical theory of all of this and discovered that a new type of wave propagation could occur, in which a changing electric field causes a changing magnetic field, which causes a changing electric field and so on and so forth. He predicted that this configuration of oscillating electric and magnetic fields would propagate through space. He was able to calculate the speed from the measured quantities that went into his theory. When he plugged all the numbers it, he got the speed of light (which was already known quite accurately). He stunned the world of physics by explaining all known facts about electricity and magnetism, while at the same time finally figuring out what light was.
Light is just Maxwell’s electromagnetic wave, oscillating at a frequency of about 500 trillion cycles per second. He predicted the existence of such waves at other frequencies. So radio waves, microwaves, infrared light, ultraviolet light, X-rays, and gamma rays are all the same thing as visible light, just oscillating at different frequencies. For example, an AM radio station broadcasts at about one million cycles per second, a microwave oven or cell phone at about a billion cycles per second.
Light, radio waves, X-rays, microwaves, etc. are all the same thing. You can call them all electromagnetic radiation to be more precise. Different frequencies of electromagnetic waves have different properties; a small portion of this frequency range (wavelengths of about 400-800nm) we call visible light. Why is this range visible light? Because that’s what our eyes can see. But visible light isn’t inherently different from any other kind of EM radiation.
Depending on how you look at things, you can view EM radiation as being either an energy wave that travels through space, or a bunch of subatomic particles called photons traveling through space. In fact, it is both of these things simultaneously, which is pretty strange. This idea was controversial until a series of experiments confirmed the ideas of quantum mechanics, which is the generally accepted theory of how very very small things work.
EM radiation can have mass in a sense; it is affected by gravity, and gravitational fields bend light rays that would otherwise travel in a straight line. (We can observe this with high-powered telescopes looking around black holes.) EM radiation travels the fastest that anything can go in the universe. Einstein’s major accomplishment in relativity was showing that nothing can travel faster than the speed of light.
Do radio waves weigh anything? They do in most conventional senses of the term weight, but physicists prefer to reserve the term weight (actually, mass) to refer to the “rest mass” which is the mass that a particle would have if it weren’t moving. Electromagnetic waves have zero rest mass, but of course they can never be at rest, since they are always traveling at the speed of light.
However, light is deflected by the gravity of stars (one of the classic tests of Einstein’s General Theory of Relativity) and, if you trapped light inside a box with perfectly reflecting mirrors and weighed the box to ridiculously high precision, it would weigh ever so slightly more.
On the surface, it’s actually refreshingly simple:
Radio, microwaves, infrared, visible light (color), ultraviolet, X-rays and gamma rays are all part of a continuum of the same fundamental phenomena called the electromagnetic spectrum. From that wiki link, you can see just how tiny the visible spectrum is in comparison to the rest of the EM spectrum.
Why we’ve drawn lines along this continuum, has more to do with the way their frequency and amplitude affect matter. But to keep it simple, it’s all the same thing which just manifests itself in different ways according to what it’s interacting with.
The problem is that this can’t be explained to a 5 year with any degree of accuracy. It can only be accurately described using high level mathematics that i won’t even pretend to understand. Any non-mathematical explanation is always just going to be an approximation of the way that light interacts with “stuff”. It will never accurately explain what light actually “is”. And the simpler the explanation, the less accurately it is.
Having said that, I will try to explain it at the level of a bright 12 year old. The following is necessarily *completely *inaccurate, but should serve to give you a feeling for how light affects things and where it comes from, which is really what most people mean when they ask what light is.
Space exists. It best to think of it as a way to describe the position of things. Two places that are close together have little space between them, distant places have a lot of space between them. That is space: the distance between things.
Matter exists. We won’t go into what matter is at this stage, but we can take it as a given that it exists.
Entropy exists. Matter wants to relax. It hates being wound up and having a lot of energy. Matter will do almost anything that will leave it with less energy than it had before.
Charge exists. Matter is made up of incredibly tiny bits, and those bits come in two basic flavours: positive and negative. We call those flavours “charges”. When matter has a charge it is unbalanced, and that lack of balance is a kind of energy. Think of it like a person carrying a heavy shopping bag in one hand or the other. It is much less energy efficient than having an equal load in both hands. So matter wants to get rid of charge. The only common way to do that is to give the energy to something with the opposite charge. So matter tries to move towards other matter with the opposite charge so it can steal that charge and balance out. And it wants to get the hell away from matter that is trying to give it more of the same charge.
So, what happens if you buy a nice new universe consisting of just space and put some matter into it? The first thing the matter does is start sending a signal out into space looking at the charges of the matter around it. If it detects matter of the same charge, it attempts to move away and if it finds matter of the opposite charge it attempts to move towards it. That signal is what is called an electromagnetic field or EM field for short. It’s a fundamental property of any charged matter, which for all practical purposes is any matter. Any matter placed into space will send out an EM field, it is the way that matter detects other charges and moves in response to those charges.
That EM field of course needs to contain all the relevant of information to allow the matter to respond to charges it detects, and to allow other matter to respond to matter that generated the field. So it contains information about, for example, how fast the matter is travelling and in what direction. After all, if matter begins to travel towards a detected charge, it is necessary to have information about the change of direction needed.
So the EM field emitted by all matter contains information about speed and direction, IOW about how much and what type of energy the matter has. Which means that the information in the EM field must also change whenever the energy of the matter changes. If the matter starts moving faster the EM field has to change to transmit this information, and similarly if the matter changes direction.
And that change in the mature of an EM field is what light is. It really is that simple. All matter emits this field that contains information about its speeds and direction. Any time speed or direction changes, the field changes, and we call that change in the field “light”.
Light is not the EM. field. Light is a change in the field. In the same way that a ripple is not the surface of the pond, it is a change in the surface of the pond, so light is the ripple spreading across the EM field and not the EM field itself…
So for example, an filament of metal in a lightbulb is cold. The matter in there is at a stable temperature, and hence moving at a stable speed. It is constantly sending out an EM field that says “I am here, I am moving at a speed of 20oC”. Because the field is stable, it isn’t light and we can’t see it. Now we flick the switch and it heats up, causing the matter to begin vibrating much faster. Now the field has to change to say “I am still here, but I am moving at a speed of 300oC”. We detect that change in the field as light. And because the matter is constantly losing energy to the room, it is constantly being heated and cooled, and every cycle of heating and cooling causes the field to change, producing constant field changes.
In really simple terms, you can think of light as ripples in the EM field that matter produces. The more energy you put in the bigger the ripples, much as a bigger rock produce bigger ripples in pond. Those bigger ripples are manifested as either brighter light (more ripples per second).
Radio waves are a type of light. They are change sin the EM field of matter. They do not have “weight” insofar as they have no mass. Like all light they have a speed, location at any point in time and a direction of travel, but these are all subject to quantum effects, so they aren’t easily explainable accurately. Basically they travel at the speed of light, in a straight line from the point of production to the observer. That’s about all we can say about any light…
And where do you fit the phenomenon of “light” into this, Im aware that light have different wavelength and so on, but what exactly IS IT?, light/radio-waves are different things on the same spectrum?
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Yes. The bigger the change in position of matter, the more information it needs to pack into the change in its EM field. Once again, the bigger the rock you throw into the pond, the bigger the ripples. The bigger the ripple the shorter the wavelength. Radio waves have really long wave lengths and are produced by really small changes. But they are still exactly the same things light waves, they are just caused by very small changes.
I will stress once again that the above is *completely *inaccurate, but it is a lie that is easy to understand and will serve for most purposes when considering what light is and how it works. If you think about it for 5 minutes you should see where it ceases to work (why light isn’t emitted when they get cold really fast for example) but for 99% of everyday explanations of why and how light is produced it should work fine.
Wait wait wait… So the speed of light has some sort of theoretical explanation? I thought the particular speed was unexplained.
For a child go with the way the education system goes. Basically you are introduced to classical mechanics up until as far as you need to go and then you are introduced to relativity and quantum mechanics. At a later age the child can read popular science books about light or study physics.
The atom has a three types of balls. Neutrons and protons in the center. Or you can pretend to the child that there is only one ball in the center and that is the nucleus. Then another ball flies around outside of the nucleus and these are electrons.
Balls need energy to move. And pure energy is light. And the light can hit a ball and it can move with more energy. Then the ball can give the energy off and the energy in its purest form travels away. That is light or a radio wave or an x ray. Then the energy can hit another ball which will travel around its nucleus with lots of energy and then gives it off again. Then the energy heads off again in a straight line. Eventually it will hit an electron in an atom in the child’s eye and their brain will recognise it.
So you have balls and energy and the energy flies around going from ball to ball giving them energy for while before the ball sends the energy onwards.
The big difference from the child’s point of view between the different types is they shake at different speeds and light is the speed that we see. Because we can see light we can all the things around us. However if our eyes only processed radiation shaking at other speeds we wouldn’t see everything. Some of them will only be taken in by metal. Imagine if we could only see metals and all the radiation hitting our eye could pass through wooden walls and we would walk into them not knowing the wall was there. Then we would bash our head. Where as because we see the light as we do the only thing we would walk into and not see is a very clean glass door at dusk (when one is slightly intoxicated but you can leave that out).
Like other answers, this leaves alot out. But there are many different levels that one can learn about light and all the matters is what level do you need to understand.
For example a structural engineer who wants a bridge to stay up would be happy with the level of detail Newtons theory of gravity goes into. But someone building a satellite would need more detail and so would learn Einsteins theory of gravity. Same with this light thing.
And don’t tell a child its too hard! Its just complicated. And make sure the child understands that complicated means it has lots of different parts that must be understood. Each part can be easy in it’s own right and when someone knows all the easy parts they can understand the complicated part. And that’s why it is hard to explain it. Rather than leave a child think maths and sciences are out of reach. They can start be learning each part and eventually the child can put all the parts together.
This is a little bit chicken and egg. The two constants alluded to above are the permittivity of free space, and the permeability of free space. The speed of light is poorly named. It is just that light happens to need to go this fast, and that gives it its name. In a sense the speed of light is explained. It is explained that light must travel at the limiting speed of space-time. However the actual numeric value isn’t explained. The values of permeability and permittivity are measures of the characteristics of space too - so they are also going to be determined by the nature of space time. So it is no surprise that all these constants are related in a fundamental manner. But these relationships don’t actually explain the actual numeric values, just that if you have two values, you know the third.
There is a speed that is woven into existence, so to speak, named “c”. It is equal to the square root of energy divided by mass, using the energy and mass values for the same thing, according to Einstein’s most famous equation. It is the greatest speed at which things in separate locations can cause one another. More often this is referred to as a constant of nature, but it has units of distance per time, or speed.
Light and other forms of electromagnetic radiation travel at this speed through empty space. If you had a magnet field meter that was sensitive enough, and somebody on the far side of the solar system flipped his magnet over so it pointed in the opposite direction, this speed is what limits how quickly you could measure the field changing. Gravitational attraction travels at this speed, so for instance if you could measure from earth the tiny gravitational fields of two asteroids traveling toward one another, this speed would limit how quickly you could tell whether they hit or missed one another through your gravity measurements.
According to Einsteinian relativity, there is no single version of “now” if you are describing two separate locations. The speed c represents an impossible limit. If an object could actually move at c, and you could also move at c, it would be possible for you to witness it getting to its destination before it leaves its origin.
So, yeah, c is a very basic ingredient of the fabric of the universe. That lay people refer to it as “the speed of light” has more to do with the history of figuring out c than it does with the real meaning of it.
thanks a lot ,great answers!
I’ll take a stab at this, from the classical side (I don’t think it’s possible to explain the correct theory, which is quantum mechanical, to a 5 year old).
Light (which includes radiowaves, the light you can see, x-rays, and so on) are vibrations of something called the ‘electromagnetic field’. This is just a fancy way of saying that there is something pervading all of space which is like the surface of a trampoline. This thing is called a ‘field’ and it can vibrate. Radiowaves are low-frequency vibrations, and x-rays are high frequency vibrations, and so on. This may sound like a vast oversimplification, but actually it is not. So what IS IT? It is a vibration in a ‘field’. What is a ‘field’? It is basically like the surface of a trampoline. It is not a trampoline, and it has somewhat different properties, but it is very similar. It is not made of atoms like a trampoline; it is more fundamental: it just ‘is’.
The more correct theory is called a quantum field theory, and is a bit more complicated. Unfortunately, unlike the classical picture (above), no one agrees on what IS IT. Some think that light really is still just a field, and the fact that it looks like a particle sometimes is just an artifact of the measurement process (for example in the ‘many worlds interpretation’) and others think that light really is a particle but where it can be found is determined by the vibrations in the quantum field (for example the ‘copenhagen’ interpretation). Since these different ideas may not be experimentally differentiated, ultimately what light really is might be a philosophical question.
We really ought to make a distinction between “the speed of light” and “c”. c, really, is just a fancy name for the number 1. For various historical reasons, humans have made a distinction between space and time, and used different units to measure them, but they’re really fundamentally the same thing, and can be measured in the same units. But since we don’t, there’s this quantity that keeps showing up all over the place.
By way of analogy, suppose that, for some reason, everyone measured the height of everything in centimeters, but made all horizontal measurements in inches. For things that were tilted, or which could tilt, you’d need all sorts of formulas to get their measurements, and you’d notice that the quantity 2.54 cm/inch shows up all over the place in those formulas. If you insist on reserving centimeters for height and inches for horizontal measurements, you’d say that this was a slope (since it’s height units over horizontal distance units), and might say that this is some sort of fundamental slope. But really, 2.54 centimeters is one inch, so this “fundamental slope” is really just a fancy way of writing “1”, and you’d never even notice it if you didn’t have this weird insistence on using different units vertically and horizontally.
It’s the same way with c. It has units of distance over time, or what we’d call a speed, so we think of it as a fundamental speed. But really, distance and time are the same thing, so we could measure them in the same units. 299792458 meters is one second, so really, c is just a fancy way of writing 1, and we’d never even notice it if we didn’t use different units for space and time.
OK, now, how does light relate to this? Well, first of all, part of Einstein’s Special Theory of Relativity was the realization that if there’s anything massless in the Universe, then whatever it is, it must travel at a speed of 1, or the math won’t work out right. Well, so far as we’re able to tell, photons (which make up light) are massless, so (as far as we can tell), they travel at a speed of 1. Now, it’s always possible that photons do have a mass, just an incredibly tiny one. In that case, they’d travel at slightly less than 1, with just how much less depending on the energy of the photon. Likewise, if the photon has a mass, then Maxwell’s equations are also ever-so-slightly off. But even if that were the case, it still wouldn’t change the fact that there’s some speed that’s equal to 1, and that that speed will show up in any equation involving both lengths and times.
Several others have already answered this correctly, but I wanted to add a few words. In Maxwell’s time, it was not understood that the speed of light was a fundamental constant, invariant for all observers, and basically a conversion factor between meters and seconds. People had been doing experiments on electromagnets and electric charges to try to understand the complicated physics of elecrtromagnetism. For example, they found that a wire carrying a current acts like a magnet. If they put it near another parallel wire carrying current in the opposite direction, the two wires were attracted to each other. The force was proportional to the square of the current divided by the distance between the wires. By doing the experiment carefully they could accurately measure the constant of proportionality and gave it a name, the magnetic permeability. Similarly they found that two objects with opposite electric charges attracted each other with a force proportional to the square of the charge divided by the square of the separation (the inverse square law). Again they measured the constant of proportionality and called it the electric permittivity. No one knew why these numbers had those particular values. They were just experimental results.
Maxwell’s theory predicted that an oscillating current or electric charge would cause electromagnetic waves to propagate away from the source at a characteristic speed. His calculations predicted a speed given by the square root of the reciprocal of the product of the permeability and permittivity. When he plugged in the numbers obtained by measuring forces between wires and charges, he obtained a number that was close enough to the measured speed of light that it was immediately clear that he had discovered the nature of light.
It took Einstein’s theory of relativity to finally show the real connection between magnetism and electrostatics. Since Einstein we understand that the speed of light is just a conversion factor between our units of space and time, but this takes nothing away from the majesty of Maxwell’s derivation.
David Morgan-Mar produces a webcomic primarily from photographs of Lego™ figures. He is also a PhD in Physics and Optics and does research for the Canon Company. He had a comic a while back (December 16, 2006), the punchline of which related to Maxwell’s equations. In the annotations, he goes into some detail about how Maxwell was able to derive the Speed of Light (c) from the equations for electricity and magnetism:
Quoth JWT Kottekoe:
Nitpick: Parallel currents attract. Antiparallel currents repel.
To answer more the OP without regard to the velocity c, here’s a simple but correct account of what electromagnetic radiation waves are:
If you rub a balloon against your hair so it gets charged, and then you hang the balloon with a weight on a long cord to create a pendulum, and set it swinging back and forth, you will create an electric field that is also shifting back and forth (for as long as the balloon stays charged and the pendulum keeps swinging).
This would create very long wavelength or low frequency radio waves, specifically 1 Hz if you got the pendulum to swing once per second. If you had a sensitive electric field meter to watch it change, and you stood off to the side so that the pendulum went left and right from your point of view, you’d detect these waves. They would be horizontally polarized, and if you wanted to make an antenna that was good for detecting them, the antenna would be oriented horizontally.
Suppose you got a little styrofoam ball out of your bean bag chair, and made another one of these devices with it, but with a much smaller pendulum that swung much faster. You might be able to get 5 or 10 Hz, maybe. Or perhaps you stick the ball on the end of a spring so it bounces back and forth much faster. If you got it going 20 times a second, it would be emitting 20 Hz radio waves, which some railroad power lines also do. You’d still barely be able to sense its movement with your eyes, I think, at 20 Hz, rather than seeing a smooth blur.
If you got the little styrofoam ball going vertically, you’d have vertically polarized radio waves. Or if you got it going in little circles, perhaps by replacing the spring with a little crank on a motor, you’d have circularly polarized waves, like GPS and many other satellite transmitters use.
The fastest motors that don’t have brushes, such as the little shaded pole induction motors you see in some small appliances, spin at 60 revolutions per second. Use that to run the crank and you’re emitting 60 Hz radio waves. Most people don’t think of this as radio, but submarines use (IIRC) 76 Hz radio waves to penetrate into the conductive ocean, and there is a huge antenna in the form of a loop of heavy cable up in the woods of Maine (or someplace up there) for doing this. They had trouble with owls mistaking the cable for a snake and attacking it, some years ago.
If you replace the little styrofoam ball with a molecular group, say a -CH3 group, and let its mass and stiff bond vibrate, which they will do because heat content of everyday objects causes their atoms and molecules to jiggle constantly, then this will emit thermal radiation, typically in the mid or long infrared. Since the resonant frequency depends on the characteristics of the molecular group, you’ll get a wavelength or frequency that is characteristic of the chemistry that is vibrating. This is the basis of infrared spectroscopy. But if there are lots of uninteresting molecular or atomic parts vibrating vigorously at a broad range of frequencies, you may just get a broad spectrum, which is how an incandescent light bulb works.
This goes on and on, but at least is a good start through radio waves to visible light.
One nitpick, there: If you’re in the same room as a 1 Hz “radio” source, then you’re not properly detecting waves. The wave description only applies at a distance from the source that’s large compared to the wavelength-- You need different formulas and techniques for the “near field region”.
Of course! Thanks for correcting my mistake.
Here’s something:
What would radio waves look like, if you could see them?
Above is from MIT electromag course 8.02, also see A VISUAL TOUR OF CLASSICAL ELECTROMAGNETISM