Science of Light

03-Science of Light

Lesson Three

Light Rays


    As we progress to higher frequencies, from radio and visible waves to infrared, ultraviolet, and x-rays, we begin to use the word “rays” more frequently than “waves”, speaking of “radio waves” rather than radio rays, and “cosmic rays” rather than waves.  The term “cosmic rays” includes not only electromagnetic waves but also a wide assortment of “pellets” composed of speeding atomic particles.  For, as frequencies increase, the waves begin to act more and more like tiny bullets, and less and less like waves.  Large numbers of atomic particles and rays go through our bodies every second.  They probably affect our lives more than we realize.


When a high-frequency electromagnetic wave (or cosmic ray) hits an atom, it seems to concentrate all its energy in one spot, enough so as to smash the atom.  A wave is more spread out than a particle, and even if it had more energy, it is difficult to see how a wave could penetrate into a single atom without affecting many atoms nearby.  But it does.  So scientists now think of light energy as coming in packages called photons.  One might think of the photons as little bundles of wave energy somewhat like the wiggly wave groups in this figure:

 No one, of course, has really seen an atom. They are too small.  Yet, not unlike a blind man tapping his way down familiar streets and even through busy traffic, scientists have formed a sort of blind-man’s picture of the atomic world.  Using theory, experiment, inference and mathematical analysis, they continue to uncover more detail.


    The arrangement of electrons around the nucleus of an atom is somewhat analogous to our solar system, with the nucleus as the sun and the electrons as the planets.  The atom’s orbiting electrons constitute a wonderfully versatile mechanism for absorbing, storing and emitting energy.  You can wind up each individual atom like the rubber band of a model airplane.  You pump in energy to wind the atom up and, as it relaxes, it radiates energy – in the form of waves.

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    Atomic emission of radiation is commonplace.  This is exactly what happens in an electric light bulb.  You wind up the atoms in the tungsten filament by heating them with an electric current.  The tungsten atoms become violently excited and collide with one another, and many of them absorb additional energy from the atoms they bump into.  This energy makes their electrons move faster and farther away from their nucleus.  The excited electrons tend to relax back to their normal state.  As each electron relaxes, it spontaneously emits the energy it previously absorbed; it emits a “bundle” of electromagnetic energy.


    To put this in simple terms: as we increase the temperature of an object, it will radiate heat.  If we increase the temperature high enough, the object will not only radiate heat, but it will also radiate light waves.  This is how the ordinary electric light bulb works.  There is a very fine wire inside the glass shell and we increase its temperature by sending electricity (also a form of radiation) through the wire.  The wire becomes very hot and radiates heat and light.  The same would hold true if we placed a piece of iron in a very hot fire.  First, the iron would become too hot to touch. If we provided enough heat (and this varies depending upon the object we use), it would eventually glow with a red light.


The “bundle” or “quantum” theory says, in effect, that although light appears to us to be a continuous stream, light appears to atoms as a hail of bullets of energy, or “Photons.”  Niels Bohr suggested that atoms exist in distinct, separate energy levels and that when they go from one level to another they either emit or absorb these bundles of energy.  An energy level has a particular orbital radius and velocity associated with it.  The higher the energy level, the greater the orbital radius and velocity of the electron.


    Albert Einstein proved by analysis in 1917 that electrons can be stimulated to jump up or drop down from one energy level or another by a photon, or bundle of energy, of just the right amount.   The amount of energy in each bundle depends upon its frequency.  Therefore the change of energy in any particular atom is related to specific frequencies of electromagnetic energy.   


    Einstein added that when a photon nudged an electron into dropping from one orbit to another, the electron would also emit another photon of the same frequency as the one that hit it; and it would emit that photon in the same direction – so that the emitted photon’s energy would be added to the photon that stimulated the action.  Here, potentially, was amplification.


  Ordinary light sources radiate light that is incoherent. This means just what you might expect – a jumble, or mixture. Light from a bulb contains all the colors of the spectrum.  Each color radiates energy at a different frequency.  Stated in terms of electromagnetic energy, it contains a jumble of frequencies.  Many waves are emitted in random directions at random times.


    In an electric light, the atoms and electrons in the filament, when heated with an electric current, (as previously mentioned), absorb energy and become excited.  As the excited atoms “relax” in a random fashion, they radiate light that is random in direction and frequency.  It is incoherent.



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   But coherent light is different.  It is of a single frequency.  All the waves move in step like a regiment of soldiers.  And all are parallel, in a narrow beam.  No one had ever practically produced such a light until the laser came along.  The exciting fact which stirs the imagination of communications scientists is that, unlike coherent light, laser light does have the theoretical capacity of transmitting voice, date and TV programs in enormously greater numbers than can be carried by present-day radio waves.


    Laser light is coherent – all random radiation is eliminated from the system and only a single frequency with all the waves in step is emitted.  Discovery of the Laser principle enabled the scientist to select a tiny bit of radiation from the visible range and amplify it into a powerful tool.


    Light waves are similar to radio waves.  Both are called “electromagnetic radiation,” but they differ in the rate at which their waves vibrate or oscillate – that is, their frequency – light waves vibrate faster, and are shorter.  Light beams have long been used to imprint the sound track on movie film and then to convert the track back into sound.  Scientists are exploring the possibility of opening up the “unused” portion of the spectrum – the infrared, visible light, and the ultraviolet regions.


    It is customary to include in the term “light” various types of invisible radiation, because their behavior in all other respects is similar to that of visible light.  On either side of the visible spectrum lies the infra-red and the ultra-violet.  Beyond ultraviolet, and of higher frequency are the x-rays and gamma rays.  On the other side beyond infra-red are the electromagnetic vibrations used in radar, radio and television.  All these radiations have in common an equal speed of propagation.  They all represent a single phenomenon, and the difference lies only in the matter of wave length (frequency), and in the energy of the photons.  Due to the enormous range of difference in wave lengths, different units of measurement are used – meters for radio waves, centimeters for radar waves, microns for infrared, and angstroms for ordinary light.  


    That there are other rays in sunlight besides the colored light that we see in the spectrum can be proved with the help of photographic film and a thermometer.  If photographic film is held just outside a spectrum of sunlight at the violet end, it becomes exposed as if light we falling on it.  This shows that there are invisible electromagnetic rays there which we call ultraviolet rays because they are beyond the violet end of the spectrum.


 If a thermometer is held on the other side of the spectrum, just past the red end, the mercury begins to rise.  This shows that there are invisible rays there that are warming the bulb of the thermometer.  Though it is ultraviolet rays in sunlight that cause sunburn, the infra-red rays make much of the heat you feel when sunlight falls on your skin.


The Composition of the Electromagnetic Spectrum:


    Radio waves cover a broad band of the spectrum from several millimeters to 5 or 6 mile wavelengths.  Frequencies range from about 30,000 to 10 billion cycles per second.  Practical use of these waves developed rapidly after they were theorized by James Maxwell in 1869.  

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The Spectrum of Electromagnetic Waves


                                                                     (Note: Boundaries are approximate

Frequency                                                               Wave areas overlap from one

(Cycles per second)                                                region to another.)

    Microwaves refer to the interval of the electromagnetic spectrum between short radio waves and the far infrared.  These waves are used in television transmission and radar.  When point-to-point communication is desired, better results are obtained in focusing at short wave lengths, particularly in the microwave range, where such effective devices as horns, metallic reflectors, and lenses are practical.

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Infra-red: The waves outside the red are even longer and of lower frequency than those of red light.  They belong to a part of the spectrum that was named the “infrared,” meaning “below the red.”  Infrared radiation is invisible, and is freely transmitted by atmospheric haze.  Infrared waves are given off strongly by glowing hot objects such as the sun, flames and electric lamps.  Even objects that are not hot enough to glow at all send out these long waves.  Electric irons, steam radiators, hot pavements and even your body all give off infrared radiation.

    Photographic film made sensitive to infrared radiation reveals objects hidden by darkness or haze.  Infrared is widely used for many heating requirements.  The common source is a specially-engineered incandescent lamp.  Certain lasers operate in the infrared range.


    Visible light: (This will be further considered in relation to the color spectrum.)


Ultraviolet radiation is found beyond the visible spectrum at its violet end, its name meaning “beyond the violet.”  This group has a save length longer than those of X-ray, and shorter than those of visible light.  It also has high quantum energy compared to visible light.  It penetrates the skin to cause tanning, and aids in the formation of vitamin D.


Luckily, the air acts as a shield to hold back most of the sun’s ultraviolet waves.  Otherwise, they would kill all living things on earth.  Space travelers need special protection from these rays.  A small amount of ultraviolet radiation is good for health, however.  The extra vitamin D in some of the milk sold commercially is put there by passing the milk under an ultraviolet lamp.


    X-rays penetrate ordinarily opaque materials, but to a lesser degree than gamma rays.  The x-ray photograph is used in medical diagnosis and the ray itself can kill cancer cells.  To create x-rays, a stream of high-speed electrons bombards a metal plate in a vacuum, disturbing the electron structure of atoms in the plate.  Energy in the form of x-ray radiation is given off from the plate.


    Gamma rays are radiation with very high penetrating power.  They are emitted by the naturally radioactive elements such as radium, and as a by-product of a nuclear reaction.  An overdose of this radiation is deadly, but medical science uses it as a weapon against cancers, tumors, and lesions.


    Cosmic Rays are a stream of high energy radiation of intense penetrating power.  Entering the earth’s atmosphere from outer space, with energies ranging from a few billions to many times that of electron volts, they bombard earth’s atmosphere to create mesons, as well as secondary particles possessing some of the original energy.  Some rays come from the sun, others come from stars in our own galaxy and a few may even be visitors from other galaxies.

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