Electromagnetic Waves

this is a radar aerial and it seems to be looking for something doesn't it how does it see an aircraft for instance hundreds of miles away and in the dark too well later in this program we shall be seeing more of radar but first here's something similar but very much simpler which is a crystal set this is the simplest possible radio receiver and you can easily make one yourselves it has no Mains connection or batteries but it does need a good aerial and an earth it makes rather a quiet sound where does this energy come from and how does it get to the crystal set I expect your thinking the answer is something about waves and you'd be right this time we're going to talk about electromagnetic waves these waves are involved in radio and television also in radar and radio astronomy and even in X-rays and in light the top is that the wave motion the motion itself is too difficult to see and even if we could somehow make it visible the frequency is far too high for us to see the waves even with a stroboscope how then do we know that they are waves well there is some evidence and I hope you all agree that it is fairly convincing evidence now our Crystal set receives wavs which are about 500 M long far bigger than this studio in fact here's a football pitch drawn on the same scale we couldn't possibly fit even one of these ways into the studio fortunately we have an apparatus here which produces electromagnetic waves which are very much shorter than 500 M and they're called microwaves this part of the apparatus is sends out the waves and it's called the transmitter and we've labeled it t the waves travel across here and they'll even go through a vacuum until they're picked up by this part of the apparatus which is called the receiver and we've labeled it R first let me show you that waves travel from the transmitter to the receiver both of these have aals on which are in the shape of horns as I expect you can see with the transmitter switched on nothing is heard in the loud speaker here until I aim the transmitter at the receiver the loud speaker here makes a note a sound wave whenever electromagnetic waves travel across to the receiver but I don't want you to be confused between the sound waves coming out of here and the electromagnetic waves coming along here so I'll disconnect this loud speaker and use this meter instead okay we get a steady meter reading as the waves reach the receiver as we can't see these waves let's look at some of the properties and see if we can recognize waves in this way sound waves and water waves we saw both went through gaps and to some extent round corners and we call this defraction let's see if it works with these microwaves here's a slit which I'll put in front of the transmitting aerial and it's so wide in fact but it doesn't cover up the aial at all now the waves received will be registered on this meter and we'll give you an enlarged picture of it straight ahead here we get a reading of about 90 round to the side the reading falls off until it's about 10 and around on this side it's down to about 15 in fact the energy does fall off rapidly around the corners as in this diagram now I suppose I narrowed down the slip it where do you think the energy would go now I want you to draw on your paper a narrower Gap and sketch in some waves to show where you expect the energy to go if we right in thinking that these waves are defected go big waves where there is a lot of energy and small ones where there isn't much energy right let's try it the other meetings about 30 here it's done only to 20 and here it's 20 as well so you should have got a diagram something like this with less energy being given out straight ahead but more spread out on either side we seem to have got defection again and some evidence of waves right here's rather a different Arrangement I'm going to put the slit in front of the receiver so that the receiver is looking through a narrow slit it's what we call a narrow aperture and now I'm going to use this receiver to find where the transmitter is as a sort of telescope not a telescope for magnifying but just for finding positions if I rotate this the reading doesn't alter very much and we'd have quite difficulty in finding out just where the transmitter was in fact it could be in any of these directions and we've got an arrow on the receiver to show where it might be now if I make this little bit wider there let's see what happens now the reading is much bigger at the moment it's about 90 and it falls off rapidly as I turn this receiver to find out where the transmitter is we naturally take the highest reading we get which is about there but still it's not a very precise position now do you remember the radar aerial well it was something like this it had a reflector and this can be used in effect as a very wide aperture aerial so that it reflects energy into the receiver AAL and of course I must turn this around and also take the slit away now we've got quite a big reading and when we're looking for the transmitter we really want to find where the maximum reading is see it falls off to 50 quite quickly on either side and this seems to be the direction in which our transmitter is and we get a very precise measurement of this with this wide aerial and we say that this aerial has a much greater resolving power another property of sound waves and of water waves which could hardly be explained unless these two motions were in fact wave motions this was called interference two waves meeting seem either to add up and strengthen each other or make and cancel out and make each other weaker here's a model to help you understand how this can happen here are two waves in Step imagine they're moving along and they meet the result is a much bigger wave and now over here we have two similar waves but they are out of step when these two waves meet we get no wave motion at all so this wave would correspond to an increased motion and this one to no motion at all here it is for sound waves on the field with the boys these boys were standing where the sounds added up to produce a maximum sound and there and in between the two waves from the loudspeakers almost canceled out and produced a minimum sound here's the same pattern with two dippers vibrating in a ripple tank do you suppose this happens with microwaves well here is our transmitter and instead of having two Dippers or two transmitters we've got two gaps here and the waves going through these two gaps we meet again at the receiver and give a reading on the meter now watch while I cover up one of these gaps cover up one Gap and the meter reading goes up uncover it cover it up and uncover it so here we have two wave motions apparently adding up to give nothing if I move this around we can find about here you've got a maximum and then around here a minimum so they cancel out a bit further along we've got another maximum and here's the minimum again on the other side we get a minimum here a bit further around and there's a maximum if I had time I could move the receiver in and out and show you that this pattern produced lines roughly along here and another one about here and another one here and here is a pattern with which you're probably getting familiar it's produced by waves coming through these two gaps and the Maxima are here here and here and now I'm going to estimate the wavelength of these electromagnetic waves by comparing them with the Shadows of the water waves on the Ripple tank I'll arrange these two gaps so that they're directly in the positions where the images of the slits are and now I'll alter the wavelength of the ripples until the strong ripples occur more or less along these lines need to be a bit longer that's about it so we get strong disturbance of the water where these black lines are now I'll measure the wavelength of these ripples I can marking it on this ruler there there there and you can see that these Shadows are just about 3 cm long just over and so this pattern on the card is made by two gaps of the spacing I had before with Waves 3 cm long and this is strong evidence that our microwaves are in fact 3 cm waves and now you probably want to know how fast these waves go well they go very fast and it's rather a delicate measurement which we can't do now but you might like to jot down the answer it is 3 * 10 10th cm/ second 30,000 million CM every second that's about 186,000 m a second and all electromag itic waves go with about this speed at the beginning I said we should see some more of radar and radar is a very important thing and it has many uses which you might like to think of we are going to concentrate on its use for navigation throughout its flight towards an Aerodrome this aircraft can be watched by this rotating radar scanner situated on a tower next to the radar control room the scanner is beaming out pulses of energy of electromagnetic waves some of the energy hits the aircraft and is reflected back to the AAL inside the control room the operator watches the screen rather like your television screen by suitable adjustments you can see a bright spot approaching the center of the screen the spot of course is a sort of picture of the aircraft's progress [Music] now the aircraft is almost overhead what Radar really does is to use the echo idea radar transmitter which is a more powerful version of the one we have been using in the studio sends out short bursts of waves they are focused into a beam with this aerial and reflector the scanner if an aircraft comes within range the scanner can be tilted towards it and some energy is then reflected back from the aircraft the scanner is also a receiver so it picks up the Echo and sends the signal to the cathod ray tube here is a scanner from above with a cathod W through screen shown as well as the pulses of waves travel out from the scanner the spot travels across the screen these waves as you know travel very fast so several hundred bursts go out every second so it's as if we were sending out a beam and this corresponds to a line on the screen but as you know the scanner turns and the line on the screen turns with it now when an airplane comes within range and the beam intercepts it a spot appears on the screen due to the echo notice that the position of the spot on the screen and its distance from the center corresponds to the position of the aircraft and its distance from the center we've made a model to summarize what is happening this is a large scale version of the screen which is of course inside the building see how the scanner and the line on the screen turn together and how when an aircraft appears the spot due to the echo travels across the screen and now it's time for you to note the questions remembering that this is the speed of electromagnetic waves what is the frequency of 3 cm waves and the second question is about the film we're finishing with it concerns ship's radar here is the scanner on the ship rotating aial [Music] and below it is the radar control room and here is the screen on the cathode ray tube the question is what do you think is being shown on this screen