Electromagnetic Waves - with Sir Lawrence Bragg

this talk is three about electromagnetic waves these waves are tremendously important I tried to say something about their nature in other types of wave we're concerned for the movement of matter waves in water sound waves where the air moves or waves on a rope or along a spring there's always matter which is moving electromagnetic waves are quite independent of matter they are the kind of waves that can exist without any matter being there for instance they heat and light from the Sun is lit magnetic waves although it's there coming across empty space to a physicist these ledge magnetic waves are almost one half of physics now Faraday intuitively recognized that there must be such things but if you had a very difficult electrical disturbance he ought to be sent out as a disturbance into space as a wave in fact but he was not mathematical he couldn't put it into equations it was the great mathematician Maxwell who first framed the right equations and showed that if there were such things as electromagnetic waves they would go at the speed of light in fact proved that light whose speed was there known consisted of electromagnetic waves what is their nature may I start trying to explain it by reminding you of two very simple effects the first one is shown by Faraday's experiment on induction here I've got a kind of wire here I've got a magnet now the effect that we're showing is this but if the lines of force convicted with a magnet are moving past a point they will set up an electrical field in this case the electrical field will try to drive account around that wire so if I move the magnet into the coil or out of the coil this emitter here connected by the wire will deflect if you watch the needle now in out in out Faraday's experiment now just notice the nature of this effect if the lines of forces are negative cut the wires this way the force is that way always a right angle the fourth electric field is at right angles to the changing magnetic field the obvious experiment is this now we'll move the electrical charges the electrical lines of force by making electrons run round this wire in other words by turning on a current there to gain produces a magnetic field at right angles the lines of force cutting this way make a feel that way when mr. Coad switches the current on we are effectively making electrons run round these coils this way the lines of force are cutting beyond this way therefore magnetic field at right angles of course is produced down this way we made it as the and and so when the current is turned on becomes quite a strong magnet and picks up all these bales now that is the essential nature of these electromagnetic waves they're suppose that this is the aerial of a radio transmitting station the transmitter is making electrons rush up and down this aerial so you've got the lines of force attached these electrons rushing up and down this way gets its magnetic field this way just as in this experiment that magnetic field makes an electrical field that makes a magnetic field and so on out goes a disturbance into space the radio station is sending out its message as electromagnetic waves here's another way of putting it with the help of this little knob what have I got here a coil in which I can put an alternating electrical current from the waves that turns that into a magnet backwards and forwards 50 times a second the changing magnetic field inside this coil makes a current run in this coil beckerson Falls 50 times the signal there's no connection Juicy's is quite loose its mary threaded by the magnetic field that magnetized is that that turns makes currents run here magnetizes that backwards opposed comes around there and so the message is passed on and if I turn on the switch at this end you will see the current in this wire by this little lamp lighting up and you will see the message is passed on to the other end and that lamp will light up to although there's absolutely no connection between the on and these corners well now I've put in coils and iron because I want the energy here all to be collected at this end and light this lamp but we could take away the coil we could take away the arm you can have an electrical field without changing without electrons having to run in the coil of wire you could have a magnetic field without there actually being any and there take away that take away that it's same time true because we have a changing electrical field here out it goes and will be picked up there only much more feebly of course for rushing out in all directions in space so that then is the essential nature of an electromagnetic wave I'll go diagram here which shows the whole range of electromagnetic waves in which we're interested here it is and it's plotted in this way one here on the diagram means one meter waves one meter long and every one of these intervals we go up a hundred times that's waves a hundred meters long 10,000 meters long a million meters long going the other way is just the opposite we go down that's one hundredth of a meter one ten thousandth of a meter right down here to 1 million millionth of a meter it's a tremendous range going from 10 to the sixth to 10 to the minus 12 a million million million times in this range round about here ten thousand thousand hundred meters long we've got long waves through radio in this region here these are shorter waves and down here where you get down to about a centimeter we have the very high frequency range of radio waves coming further over we get vocal infrared rays heat waves the waves whose warmth we feel when we stand in the Sun and when we get down to about one millionth of a meter we begin to see red waves have a wavelength slightly less than one millionth of a meter and the visible spectrum from the red to the blue which I've drawn here in color extends over about one octave blue light is about half as long as red light beyond death is also light but we can't see it that's called the ultraviolet and beyond that again we get still shorter waves soft and then hard x-rays and then what's called gamma rays radioactive substances and beyond that again still shorter ones that come in the cosmic rays so there then is the range of a little magnetic waves that we are going to consider I've got here well that's called a klystron which is sending out a little bit it equates just under one centimeter in wavelength out of this little horn here at the other end here there is a receiver which picks up these waves and the pick up is shown both by this little lamp here lighting up and by a note I might say but the note you hear is certainly not the frequency of these ways which is of course very high indeed these are modulated as the electronics engineer calls it to give an audible sound so when this receiver is picking up and the waves you hear you hear a note and you see that little bulb light up if mr. Coates switches on you will see this horn receiving the waves by this lamp lighting up and by the noise you can see the waves are coming from there I can cut them off with my hair now reflection if I move this arm round over here I've now moved it of course out of the out of a beam and so it's picking up nothing but if I take a mirror like this I can throw the signal back into it by means of this reflecting screen here piece of aluminium their art is user fiction it's very sharp reflection reflection here's a prism made of paraffin wax is transparent to these ways although of course isn't transparent to light the waves go slower in this prism and they do in the air and therefore if I sit that now on my table here and move this round I hope will pick up we reflected waves through that flock AR Z now we can turn this little you see it says to dress the position of the prism and the position of the arm polarization now this is an interesting effect we've seen that these electromagnetic waves the magnetic and the electric fields are at right angles to the way in which the waves are traveling in this particular case this klystron is sending out waves which the electric field is up and down I can show that by means of this screen here I'll move this round first of all but it picks up the direct beam and here I've got a screen of conductors now if I place that in such a way that these conductors are parallel to the electrical wave they kill it as it were they being conductors are countless in them to kill the phone electric polarization so you will see if I hold this with the rods vertical it cuts it out if I turn it round and hold of horizontal it goes through vertical horizontal let's show you see that the electric vector in this case is in this direction at right angles the propagation of the waves focusing with a lens here we've got a perspex limbs mr. Coates is going to move that receiver back so it barely here's the waves from this klystron but if I put the lens here in the path of the Rays it will focus the electromagnetic waves onto that receiver see take the lens away and we don't get the single again all focusing with a concave mirror the coast has here a concave mirror and this time I'm going to pick up the reflected being able to look for refocus by means of is similar receiver there is a little horn which will pick up the waves which I can move about now the waves the coming out of this cinder they have been focused somewhere out here right that concave mirror and I think I hope I'll be able to find the focus by feeling for it with my receiver there it is you see I'm passing through it and about that point there so you will see that this concave mirror focuses the electromagnetic waves somewhere out here just as in the case of light well now I'm going to repeat with my electromagnetic waves the very famous experiment of Thomas Young the pinhole experiment when he got the interference fringes which proved that light consisted of waves in the case of Young's experiment he had to put the pinholes very close together indeed he had to look at the fringes which were still quite close together at quite a distance away because in that case the wavelength of light is so very short indeed with these centimeter waves we can do everything on a much coarser scale mr. Coates will put two slits in front of the beam of waves here they are these two slits in this screen and I'm going now to pick up what I get on the far side with this receiver which I can move about first of all I will cover up one of the slits like that and now if I search across the field with my receiver you will see the just is a patch a single patch of waves coming through that slit so but now if I uncover this lid now we get two sets of waves coming through and we will see the interference fringes between those two six as I move this across sigh pass through one to set one process out there so the interference is taking place between these waves well finally I want to show you an experiment which measures as it were the length of these waves may I first remind you what we mean by standing waves and I can demonstrate that again by means of our binnacle model which I've got over here here is by finicum model and I can send waves along it by oscillating this rod at one end now why won't you notice is this but if I send a series of waves along this model and the reflected waves meet the uncoming waves may produce a series of nodes and loops nodes where the rods remain stationary and loops where they're oscillating backwards and forwards and if you keep in mind the scale of the waves I'm sending along and look at these nodes and loops you'll see that wave length includes two loops is the distance between alternate nodes well now I'll start by sending a series of rather short waves along and you see these nodes and loops now they're going to be reflected do you see how it's beating in this series of nodes and loops well now we repeat it with rather longer waves I'll ask mr. Kurtz to Dan pal model heard I start with it still again now I'll Sint rather longer waves the log and of course in this case the nodes and loops will be unbothered a larger scale watch now well now I'm going to illustrate the same principle with my electromagnetic wave setup over here here again is my cinder which is sending out these electromagnetic waves if we take the double-slit away and put at the end here a plane reflecting mirror now we've got the Train of waves coming along from the center here reflected by that mirror and interfering making beats between the two sets of waves going that way and this way in this case the waves are rather less than a centimeter apart in fact this is an experiment to measure the length of the waves and as the waves are about a centimeter long the beats will be half that apart actually about four and a half millimeters so I've gotten to put them rather steadily and I put my hand on the Block like this and move this very steadily backwards and forwards I hope you're still passing through the nodes and loops so you'll see they're about half a centimeter apart I heard chattering as I go quite fast so there you see we are actually observing that this what we saw with the vanakkam model and of course is there is phenomena but I've been illustrating with these centimeter waves are characteristic of the much shorter light waves and of the x-rays which very much shorter still you you

Channel: Ri Archives

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Keywords: Science, vintage, classic, old science film, black and white, archive, experiments, Faraday, Bragg, Lawrence Bragg, classic experiments, heritage, crystallography, archive footage, physics

Id: Vwjcn4Vl2iw

Length: 20min 23sec (1223 seconds)

Published: Mon Oct 24 2016