Of Science and Scientists; 17; Light: Waves or Bullets?

What is this the effect of a stream of particles shooting through space like little machine gun bullets. Or is this a procession of fast moving waves. Or are they both here. And are you really watching the light at all or merely where it comes from. Watch as from these apparent contradictions. Science finds a universal property of all forms of existence. You. It should go without saying. But

to say she contradicts herself would be to say something that has no meaning. They are times when Nature seems to contradict itself. When we find the nature of facts that seem utterly inconsistent these are the paradoxes of nature. They are fascinating to the scientists. They don't challenge his understanding. It all began with the problem of the nature of light by like the sea there were eight docking rays we mean chain that continually living in contact with the world around us like from the sun and the accompanying invisible light rays of radiant heat bathed the area and make it warm and habitable. Just what is life. Since the 17th century there have been two competing theories regarding the nature of life. According to the corpuscular theory a beam of light in 16 tiny particles are

causally ejected by the light sauce whether it be the sun or a star or a man made land. It guarding the wave theory on the other hand a beam of light is a street art train of high speed ways that are radiated by the lights are such as sound waves radiated by a vibrating tuning fork. To illustrate the problem let me form a beam of light. But placing this sheet containing a round hole in front of a small and brilliant source of light. Now I notice that you cannot see the bean and ice you scatter some chalk dust across its path and so scatter the waves into your eyes. Or you can locate his path by intercepting it with a sheet of cardboard.

You can't tell where the light is like machine gun bullets are like a train of waterways by merely looking at it. The reason is that the Asian consists not in seeing light but in seeing where it comes from. We're all long by the rays of the sun so a light must carry heat energy with it from place to place. It travels at enormous speed in lines that are generally straight. This could be done of course by applying causes like machine gun bullets as in this diagram but it could also be done via train or stream of waves. No other plausible interpretation has occurred to us that the question is. Which is it. Ways are fossils. This question is argued by big Newton and Christian Huygens. Back in the 17th century Newton took the corpuscular point of view that Huygens preferred the

way theory in the 19th century new facts about the behavior of life were discovered that seemed to decide the question once for all in favor of Huygens in the way of conception that paradoxically built on the fact discovered in our own century that the largest to revive the corpuscular point of view in Newton's opinion the fact that light travels in straight lines and cash job Caddo could only mean that it is corpuscular that demonstrate this casting of shop shadows. Water waves and sound waves bend around corners. They do not lend themselves to the casting of shop shadows. Though Newton argued that light cannot be a wave motion. This argument however is convincing no longer. Today we know much more about wave motions than Newton did.

They discovered for example that when a train a parallel waves strikes an opening in the barrier the extent to which you the bends around the edges of the barrier depends both on the wavelength and on the size of the opening. For example here is a picture of a train of ripples passing through an opening in the barrier. The dotted lines show the edges of the being that would be for long. If the wave cut off sharply but as you can see the way spread beyond the dotted lines just the same the train of emerging Wade does a crude bean. Now here I've marked in a single wavelength running from the crest of one wave to the crest of another. By sharpening this wave length you can sharpen the bean causing it to remain more closely within the dotted lines. These of course were ripples on water like way down much shorter

but actually even a beam of light spread of it at the edges and cast shadows that are not perfect the shop. In fact the spreading of light waves the wave theory perfectly once we understand very sharp light waves are so. Newton's objection based on the shop shadows it turned into an argument in favor of the wave conception instead of the corpuscular conception. There are many such arguments but the strongest argument by the way Theory of Light is the fact that overlapping beams of light can produce the phenomenon of interference. Suppose for example that we have a thin sheet of metal in which two parallel slates have been cut. Such as this. Imagine two sides to that placed in front of the beam of light from a distant small saw. If the slits are narrow enough and they must be very narrow

close enough together the two slender beings emerging on the right will spread it right enough to overlap each other by intercepting the light with a cardboard screen or a photographic plate you can study the distribution of light intensity in the region where the two beams overlap. This is what you see on the screen are developed plate a pattern of vertical stripes either of the two overlapping beams by itself would produce a simple patch of light like this. But when the beams overlap that just crossed by darks drives the two beams combine in some places to produce documents. This we call interfere it is a mutual cancellation. The two overlapping beings that take place in certain narrow zones of the overlap region. Such a

cancellation is hardly conceivable. If we think of the two beams the two streams of machine gun bullets the streams of fire from two machine guns always reinforce one another. They can never cancel out. On the other hand this interference is exactly what we would expect on the way theory. For example the same thing happens with the ripples on the surface of water. Here there is a shallow pool of water from a lamp below in such a way that we can see by projection what happens when the water is agitated. The agitation is accomplished by a metal finger which can be made to vibrate in the water. Let me start it. You can see a regular Crane of expanding circular waves with a definite wavelength a definite distance between the crest of successive waves one vibrating thing or one set of ripples. But now suppose

you two. When both fingers vibrate oscillate in unison they produce two similar systems that cross one another as they expand. You can see on this now on this photograph Jones a strong wave motion and between then lines like the aisles in an amphitheatre where there's a little R No wait wait no wave motion. Wot in the zones where their strong wave motion the crest that one waits for a neat the crest of the other in the same zone as the trough of one system neat soft of the other. Here then the two wave trains cooperate but along the lines that mock the aisles of our amphitheater the crests from the right and a trough from the land so the do motions cancel one another. And here we have interference. Now in the case of optical

later Phares two slits much smaller and closer together. Then DS take the place of the two fingers dipping into water the same lofted in the zone of cooperation and interference exists in the overlap region. But to make them visible in this case we must catch the light on the screen or a photographic plate. The existence of such alternating bright and dark bands shows that light does have the properties of a wave motion. The spacing of the band depends on the distance between two successive. In other words on the wavelength using such interference bands. Scientists can actually measure the wavelength of light. It turns out to be extremely small about a 50000 part of an inch for green light. This is much smaller than the smallest dust particles you can see with your naked eye but much larger than for example the diameter of an atom. No ordinary white

light is a mixture of many different colors and corresponding wavelength. You are there I am sure. The effect produced by a prism when placed in the path of a beam of light. When light light strikes the side the rays are bent toward the base the shorter waves are bent Mark. And are bent more than the longer ones and this causes the separation of the different colors without color television and color television you won't be able to see the resulting rainbow effect like. Is the band of color on the screen that passes for red on the right through our range yellow blue green divide. The colors are arranged according to their wavelength. The red on the right being the longest. But this is by far this is far from the whole story.

In addition to the wavelengths to which our eyes are sensitive there are other wavelengths invisible colors so to speak that lie the on the violet end of the visible spectrum. These are the rays that produce sunburn and beeches other invisible colors with wavelengths longer than those that visible light can be detected beyond the red end of the spectrum. Out here these are classified as infrared light or radiant heat. These infrared rays are primarily responsible for the water glow from the fireplace. But this is not all. We have gradually discovered how to generate a steady radiant energy with all wavelengths from several miles to a fraction of a billion to the new age. Here's a chart that cover that shows them all on one end we have radio waves which may be as much as a mile long. Then we have microwaves. Centimeter waves using radar then the infrared heat waves in this narrow visible spectrum of the red here and blue over

there. Divided lie then x ray region and finally gamma rays where the wavelength getting shorter and shorter as you go across the diagram. Now all these different classes of radiant energy are generated in different ways though they differ from each other in some respects. They are alike in for example all travel through a vacuum with the same speed one hundred eighty six thousand miles a second. And in each case from radio waves to x rays the enormously different wavelengths measured by a suitable adaptation of the same method of interference. But now after all the build up about the way theory here's the paradox. There are still other facts about life and other short wave forms of radiant energy that seem to contradict the wave conception flatly. These other facts had to do with the way in which dark ways interact with matter. For one thing it is necessary to use increasingly violent methods to generate

radiant energy of shorter and shorter wavelengths. A poker heated to a moderate temperature goes dull red it emits almost no blue light to generate the temperature and to be stepped up sharply and to generate ultraviolet light. We have to use the violence of a heavy electrical spot. Let me show you how to produce x rays with even shorter wavelengths. We have to fall on the beam of high velocity cathode rays in a vacuum tube and use it to bombard a block of heavy metal. The greater the energy of the cathode rays the shorter the wavelength of the X-rays they generate. Violent effects are evident when shark ways are absorbed by matter and that by this I mean waves all the way from the visible through into this gamma ray region. Ultraviolet light and x rays and eyes the atmosphere that is they tear apart the molecules of oxygen and nitrogen that make up the air flowing positively and negatively charged fragments called ions.

The longer waves of visible light and infrared light cannot do this nor can the longer wavelength produce sunlight that comes from the ultraviolet in the sun's rays. An overdose of x rays the dish super sharp wavelength quickly produces much more dangerous brain. But although these indications of violence are strikingly dependent on measured wavelength they're not easy to understand in terms of the wave theory. There's no apparent reason why waves of high frequency and charge wave length should be harder to generate Margaret struck data than those of long wavelengths. We can delve a little into these remarkable properties of short wave radiation by means of a technique very used in the study of the so-called alpha and beta rays. These are high velocity particles and that are given off by radioactive substances Bundy's rapidly moving particles pass through the air to ionize it just as X-rays do.

It binds up on the air saturated with moisture the the water tends to condense on the ions in tiny droplets in a specially designed cloud chamber as it is called. It is possible to convert an invisible trail of Jardines into a visible track of water droplets. This visible track is not unlike the object that they hide by a speeding jet plane on a clear day. Here is a photograph of a sheet of Apple dragged from Polonium each track is composed of many tiny drops of water. You can see that most of these tracks ended about the same distance from the radioactive saws. The Razor slowed down as their energies use in the production of ions and each drag ultimately terminates at the point where the alpha particle is brought to receive such a cloud chamber photographs reveal many sorts of events in the in the atomic where for example. Here is an alpha particular sharply deflected by a collision with the nucleus of a chlorine atom.

So far you have seen all the tracks produced by off a pot of gold on the other hand beta rays. Electrons are much lighter and hence more easily deflected. Here you can see that their tracks IME are irregular than at the track. However very fast beta particles do as straight and very thin tracks such as you see here. So much for the effects produced in a cloud chamber by alpha and beta particles. But what happens when we send a beam a short wave length radiation through a cloud chamber. For example a beam of light like but invisible x rays the ions produced by the x rays strung along trailers directed like attractive out of bottles along the path of the bean. If this were the case it would prove that x rays are like the corpuscular rays of matter. But since x rays have wave properties like light we do not expect this.

It would seem more likely that the ions produced would be scattered over the entire space so that by the X-ray beam. Indeed this can happen as you see here. But to get this effect we would use x rays extremely long wavelength. It was a shorter wavelength of x rays. This is the result. Shorten the wavelength filter either and this happens again. Then one gets a picture of this guy clearly shorter X-rays do generate tracks but these tracks are not lined up in the direction of the beam. They star in the path of the being in the up and down. Compare the appearance of this x ray effect with this the track body you see the similarity the tracks left by the

x ray suggest that the pot of gold that produced them are of the same nature as beta particles they are in fact attracted electron the ejected from air molecules with sufficient speed and energy to form trails. We call these photo electrons from the point of view of the wave theory of light. These thought and I ground the electrons ejected from atoms via light very remarkable. The energies of the individual photo electrons are unaffected by variations in the intensity of the x ray waves. However if the wavelength of the x ray beam is sharpened the energy of the electrons is increased merely by shortening the wavelength and increase to any desired amount. This is not the kind of thing we would expect from the wave theory of x rays a wave motion could not of itself provide the concentration of energy needed by such a violent thing. In fact the

phenomenon suggests that x rays are not waves at all but a barrage of explosive shells or packages of energy that can explode in an atom and deliver their hall energy content through a single electron thus turning it into a photo electron. In fact there seems to be No it's plate escape from the conclusion that x rays have corpuscular properties as well as wave properties like Dr Jekyll and Mr Hyde. They have a dual personality and this is true for x rays. It must be true for all other pharms radiant energy from one end of the chart to the other. Therefore we assume the existence of energy puzzles in radiant energy and we call them photons. Remember they are not the same as photo electrons. They are the explosive shells which produce thought of electrons by measuring the energies of POTO electrons whose tracks we see scientists can work backward to infer the energies of the photons that leave no tracks. These

energies increase as the wavelength becomes smaller and smaller. Now think back a minute or so to the explanation of the production of x rays. You can see now why it is that high velocity cathode rays are needed to generate sharp wavelength x rays with their powerful photon energies to create these high energy photons you must have cathode rays of equal or greater energy. You can also understand why x rays and ultraviolet light are dangerous when they fall on living cells their photons produce both the electrons just as they do in air. If the cells are in our own muscular tissue the thought of it like Brown's pass through the tissue and produce destructive ionisation wherever they go. Now you see the paradox on the away theory can explain the interference experiments by which we measure wavelength

on their car a particular theory can explain the explosive the direction of thought and ways are conceived to be continuous disturbances so that energy is thinly spread out in space whereas photons are like our fossils flying pinpoints of concentrated energy. Common sense tells us that life must be one thing or the other. But the facts will not have it that way. That nature contradict yourself by no means what you does it to force us to conceive of light and other forms of radiant energy as something that spreads out in space as if it were a wave wave motion but which interacts with matter as if it were a barrage of explosive shells. Perhaps we should think of ways as guiding core puzzles in any case. We must certainly retain parts of the wave conception and parts of the corpuscular conception or a complete interpretation of all the facts. No

it furthers this uncomfortable hybrid point of view met with strong resistance among scientists. However this resistance was very very due when it was shown plenty years ago that electrons in atoms have the same paradoxical combination of corpuscular properties and wave properties that light and X-rays have. If this set up is used to send a beam of x rays through a box of powdered crystal The Rays are scattered in directions which make well-defined angles with the beam if the scattered rays are caught on the film they produce a pattern of content rings like this the pattern is due to interference between the waves scattered by the atoms in the crystal lattice. We can use this pattern to measure the wavelength of the X-rays. But enough to blow us up to do it. A suitable pencil moving

electrons for the x rays and a set up of the saying in this case to produce a similar pattern of content free range. This too is an interference pattern and interference pattern produced by the electron compared with the one produced by x rays. The similarity is that electrons in motion like x rays behave as if they were guided by ways we can measure the wavelength by the pattern and show that the relation between energy and wavelength for electrons is in harmony with the relation between energy and wavelength for x rays and for light. So all the dualism of wave and particle is not just a property of light. It is a universal property of all forms of physical existence. The universality of the padlocks makes it easier to accept. Moreover the discovery that matter has had wave properties that turned

out to be the real key to the understanding of the structure and properties of the atom atoms only make sense when we understand that each is both a system of potholes and also a complicated system of standing waves like those in a bowed violin string. The discovery and acceptance of this universal dualism of wave and causal is the heart of the great revolution in modern physics and chemistry that we call quantum theory. These ideas of course outrage our common sense but they did all the known facts that where the modern physics is built upon them. Most of the boxes of modern physics. The iPhone this that weekend each Indian measurement physical quantities that on the outside expiate they are not at our skate. They are much much too small. The smallest of it I like. It's quite beyond our

imagination to conceive yet we can find out within limits how an electron behaves after we have found that out. We can go to an electron beam so that it places on a TV to be changing down which you are watching at this moment. It is tough to accept that we can control something we cannot imagine yet such is the case in electronics. Science is an experience that man has never had time. That is why it is so upsetting. It is all so powerful. This is National Educational Television.