Physics Help Forum How can we see things that don't emit photons?

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 Apr 16th 2014, 01:03 PM #21 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,352 OK, let me try. First, about waves. A wave, like you see on the surface of a lake, is characetrized by three parameters: wave height (or amplitude), wave length (distance from one peak of a wave to the next peak, which we'll denote as L), and frequency (f, how many waves go past per minute ). Two other parameters that are related to these: period (T), which is the amount of time between wave crests (and is the inverse of frequency), and speed of the wave (v) - how fast it travels acrioss the water's surface. There is a relationship between speed, frequency, and wavelength: v= Lf, or v=L/T. Besides water waves another type of wave you may be familiar with include a vibrating string (like the guitar string mbw mentioned earlier). A guitar string has a wavelength set by the guitar player's finger, and it vibrates at a rate that is dependent on the material the string is made of and how taut it is: a lighter struing vibrates quicker than a heavy string, and a taut string also vibrates quiker than loose string. On a guitar there are 4 or 6 strings, each set to a different tautness in order to cover a different range of tones, and the player controls the length of the vibration by pressing his finger against the string and clamping it to the fret board. The frequency of the sound you hear is determined by the above formulas: f = v/L. Now for electric and magnetic fields. An electric field is a measure of the potential to move a charged particle. Imagine two parallel metal plates, with one having a positive electric charge and the other a negative charge. We can do this by connecting the positive terminal of a battery to one plate and the negative terminal to the other. If we place a free electron between the two plates it will try to move toward the positive plate and away from the negative plate. The force that the electron experiences is measured in newtons, and is related to the voltage that is applied to the plates divided by the distance between the plates. We describe the magnitude and direction of this force in terms of volts/meter. For the case of the parallel plates the electric field is perpendicular to the plates, by convention pointed in the direction of the negative plate. You can get different configuratons of electric fields by changing the geometry - for example a positive point charge all by itself has an electric filed that radiates in all directions from it. A magnetic field is sort of similar, in that it's a measure of the force that either end of a small dipole magnet feels. Its direction is defined as the direction in which a small dipole magnet must face for it to not experience any torque from the magnetic field. In the 1800's it was discovered that if you place a wire in a changing magnetic field you can create a voltage along the length of the wire. One way to do this is to move the wire across a constant magnetic field. This is how a generator works - by rotating a coil of wire around magnets it generates voltage. Stated another way - a changing magnetic field causes an electric field to be generated, and it turns out that electric field is oriented at 90 degrees from the magnetic field. Also in the 1800's it was discovered that current in a wire causes a magnetic field. For example if you wrap a wire around an iron nail and apply a volage to the wire you create an electromagnet. A scientist named James Maxwell took these ideas, along with a couple others, and was able to develop a system of equations that describe how light is actually an electro-magnetic wave composed of an oscillating electric field (which being a wave has a wavelength, amplitude, and frequency as we described before) and an oscillating magnetic field that is rotated 90 degrees from the electric wave. As the electric field changes in magnitude the magnetic field also changes (remember how a changing electric field creates a magnetic field), and also as the magnitude of the magnetic field changes it changes the strength of the electric field. Thus the electric field and magnetic fields reinforce each other, and the result is a traveling wave of electromagnetic radiation - if you watched the animation I pointed you to earlier you can see how the perpendicuklar electric and magnetic fields interact. These waves are very, very small - the wavelength of visible light is about 5x10^-7 meters. And they move at the speed of light, 3x10^8 m/s, so the frequency of visible light is about (3x10^8m/s)/(5x10^-7m) = 6x10^14 Hz. One significant difference beween light waves and the mechanical waves I described earlier (water waves and a vibrating string) is that light does not have a medium that it travels in. To have sound waves you must have air (or some other medium), to have waves on a lake you must have water. But light is effectively its own medium - there is no substance that it travels along. It is self-contained. I hope this helps explain how light waves work. Notice that there is nothing here about photons. This is because the particle theory of light doesn't care at all about waves and electric or magnetic fileds - it assumes that light consists of tiny particles of energy called photons. When I say "tiny" I mean really tiny - in many ways photons act as if they are infintely small. This is the really weird part - the dual nature of light means it's a wave and it's a particle, which is something that in my opinion is impossible to visualize. Hope this helps!
 Apr 21st 2014, 07:26 PM #22 Junior Member     Join Date: Apr 2014 Location: India Posts: 5 According to maxwell"Electromagnetic radiation (EM radiation or EMR) is a form of radiant energy, propagating through space via electromagnetic waves and/or particles called photons. In a vacuum, it propagates at a characteristic speed, the speed of light, normally in straight lines. EMR is emitted and absorbed by charged particles. As an electromagnetic wave, it has both electric and magnetic field components, which oscillate in a fixed relationship to one another, perpendicular to each other and perpendicular to the direction of energy and wave propagation."(Source Wikipedia).For more of it http://en.wikipedia.org/wiki/Electromagnetic_radiation Electric field and Magnetic field and direction of propagation are perpendicular to each other in a EM wave.I have enclosed an attachment showing wave propogation in medium. Attached Thumbnails
 Apr 26th 2014, 12:47 PM #23 Member     Join Date: Feb 2014 Posts: 48 Ok. Thanks. How is the wavelength of light measured? So, the magnetic field of light is measured by a magnet? How exactly is it measured ? Does a lightbeam have a magnet field ? And An electric field is a measure of the potential to move a charged particle. So does lightbeams have the force to move charged particles? And also a question about light. How can It be waves, when a lightbeam foes straight forwards. A wave doesn't go straight forwards, it goes outwards in every direction. Thanks __________________ Last edited by Anonymous; Apr 26th 2014 at 01:47 PM.
 Apr 26th 2014, 08:06 PM #24 Junior Member     Join Date: Apr 2014 Location: India Posts: 5 It was pointed out later by De broglie that light shows wave particle duality.There was a giant debate between Newton and Huygens over nature of light.Newton had his corpuscular or particle theory and Huygens had his remain Wave theory but due to respect for Newton,Wave theory remained in trash for almost a century.Thomas Young in 1801 by his Double slit experiment and Grimaldi by his Diffraction theory resurrected Wave theory.It enjoyed unquestionable status for many years and meanwhile came Maxwell's Radiation about which i explained you in my previous post. You must try learning Maxwell's equations they may answer your queries about magnetic waves. Monopoly of wave theory was broken down when first photoelectric effect experiments were done. Wave length of monochromatic light may be measured by Michelson Interferometer or you may calculate frequency and use expression(There are many methods to do it). f(frequency)=c(Speed of light)*(wavelength) http://en.wikipedia.org/wiki/Wave%E2...rticle_duality http://en.wikipedia.org/wiki/Matter_wave Wikipedia links provided explains everything i guess you need to know.
 Apr 28th 2014, 05:08 AM #25 Senior Member     Join Date: Apr 2008 Location: Bedford, England Posts: 668 Electrical and Magnetic interactions between objects drop off rapidly with distance, however they never reach zero. Thus all charged particles in the universe are simultaneously pushing and pulling agauinst each other. The overall sum of all these interactions is called the electro-magnetic field. In "normal" conditions the electromagnetic field is almost exactly in balance. However, any movement of charged particles upsets this balance. The effect of this initial disturbance creates a distortion of the electromagetic field that can be described using a set of mathematical equations that were worked out by James Clarke Maxwell. These equations have very close similarities to the mathematical equations used for describing waves (e.g. water waves, or sound waves). Hence this disturbance in the electromagnetic field is called an electromagnetic wave. However, as you correctly surmise in your post, a wave would normally be expected to expand equally in all directions while light is often regarded as rays. I think this is due to the way light is generated (and the way is is absorbed). Radio waves and light waves are fundementally the same, a disturbance in the electromantic field, the difference is in the wavelength (frequency). However radio waves are rarely envisaged as rays, they expand away from the transmission arial in a "properly" wavelike form. Radio waves are generated by large groups of electrons moving in concert. Light however is generated when individual electrons emit a very specific amount (quantum) of energy. Thus the electromagnetic disturbance can only contain this amount of energy. This precisely bounded quantum of electromagnetic disturbance is called a photon. Last edited by MBW; Apr 28th 2014 at 05:11 AM.
 Apr 29th 2014, 10:41 AM #26 Member     Join Date: Feb 2014 Posts: 48 So, wave length of monochromatic light may be measured by Michelson Interferometer. How is the electric and magnetic field measured? Can you please try to explain in simpler words what the electric and magnetic field is. Is it something that one can measure where there is light, like in a light room? Thanks __________________ Last edited by Anonymous; Apr 29th 2014 at 10:51 AM.
 Apr 29th 2014, 12:51 PM #27 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,352 As noted previously - when using the term "light" you must be clear as to whether you mean electromagnetic radiation in general, or visible light in particular. I am not aware of any instrument that can directly measure the electric or magnetic fields of visible light, though obviously we can measure related characteristics such as light intensity. The problem is that the mechanism used to measure an electric field must have an antenna that is no bigger than approximately 1/2 wave length of the electromagnetic field being measured. For radio waves - which have wave lengths on the order of several meters - there are many commercially available electromagnetic probes that can do this job. But the wavelength of visible light is so small - on the order of 10^-7 meters - and the frequencies so high that no mechanical device has been devised that would be small enough to measure the electric or magnetic fields directly. Last edited by ChipB; Apr 29th 2014 at 12:54 PM.
 Apr 29th 2014, 02:41 PM #28 Member     Join Date: Feb 2014 Posts: 48 Ok. How does the Michelson Interferometer work? Thanks __________________
Apr 29th 2014, 07:11 PM   #29
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 Originally Posted by Anonymous Ok. How does the Michelson Interferometer work? Thanks
Try watching this video

http://video.mit.edu/watch/michelson...erometer-6561/

It demonstrates measuring of wavelength of laser light by Michael Interferometer

http://en.wikipedia.org/wiki/Michelson_interferometer

Hope this helps..

 Apr 30th 2014, 05:18 AM #30 Member     Join Date: Feb 2014 Posts: 48 ChipB: You wrote that in summary the lamp emits photons of essentially all visible wavelengths (white light); some of those photons are absorbed by the chair and some are bounced to your eye, with the result that you see the chair's shape and color. So a wave in the wavetheory of light also consists of photons? A wave of photons? Thanks __________________

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