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Quick question on interference of light

  1. Dec 4, 2012 #1
    I've studied Young's slits and other diffraction type experiments and so understand how a light wave can interact and constructively or destructively interfere depending on path difference etc. My question is: why is this not more obviuos is everyday life?
    Say i'm looking at a cup placed on a table a few meters in front of me. I can see it because light waves are moving from the source, reflecting off the cup and into my eyes.
    Now say a person shines a very powerful laser at right angles to me and the cup (in a room with no dust or other particles in the room for the laser light to reflect from) then my view of the cup will not be disturbed at all. The person with the laser could move it away from perpendicular, yet still shining across the path between me and the cup and still nothing.
    Is this due to the nature of light being of more particle nature in this case?


  2. jcsd
  3. Dec 4, 2012 #2


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    Consider a wide single slit: Every path which deviates significantly from a straight line through the slit has a very low intensity - you get so many different phases from different points in the slit that the intensity is low everywhere. In everday setups, you can consider everything as extremely wide slit - a window, for example, has a width of ~1m. You need very special setups to get all phases to align anywhere but in a straight line from the light source.
  4. Dec 4, 2012 #3


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    I'd say that it's more to do with the wavelength to which our eyes happen to be most sensitive and the relative size of everyday objects. Ray optics is easier to describe in terms of waves than photons as it follows perfectly the predictions for wide apertures. (My strong opinion is that you should avoid the photon model in nearly all cases until you 'really really' understand it because it will turn round and bite you.)

    I was one of the last few cohorts of University students to have been presented with Laser Light as something 'special' and I remember the 'speckle' being very hard to come to terms with when I first saw it. That's probably the most obvious diffraction effect unless you are into astronomy or microscopy, where it hits you as soon as you want to get good images of tiny or distant objects.

    If you spend your life being aware of the practicalities of radio communications then the effects of diffraction (two slit interference is just a simple example of this) are very very obvious. Every directional antenna (dish or multi-element array) works according to diffraction theory.

    Diffraction effects are easily detectable in the context of hearing sound - although our brains neatly tend to edit them out (and even use them to advantage).
  5. Dec 4, 2012 #4
    What you are pondering has to do with a very special property of waves, something that sets them apart from matter. If two pieces of matter "interfere" with each other, two cars trying to go through the same intersection at the same time, they have a lasting effect on each other. Their direction of motion is altered and their fenders get crumpled.

    When waves encounter each other they "interfere" with each other while they are moving through and sharing the same space, but are not otherwise changed. After they pass each other they look and travel exactly the same as before the "interference".

    Fill a big cookie sheet with water. Simultaneously dip two fingers into the water, one at each end, and watch the waves carefully. Initially, you have nice circular waves spreading from your finger. Next you will see the interference pattern as the circular wavefronts collide with each other. Finally, you will see the smooth circular wavefronts emerge again beyond the collision.

    This is why your 90 degree laser light does not effect you perception of the cup. There was interference between the light from the cup and the laser light while they both shared the same space, but the laser light went on its merry way leaving the light from the cup undisturbed to be seen by your eyes.

    As the previous poster mentioned, this has nothing to do with photons. Evidence of the particle nature of light is not detectable to our bare senses, it requires apparatus.
  6. Dec 5, 2012 #5


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    Whenever we look at anything, there is light energy travelling across the space in between - the sun, light from the sky and clouds etc. etc.. It is only when a wave actually enters our eye that we 'see' it. We say that space, air, glass are all 'linear' media. That is to say, the value of electric field is the sum of all the contributions of fields at that point. This means that EM waves do not effect each other as they pass through a medium. Interference is not something that occurs actually in the air but at the point where it is measured - say the screen in the two slits experiment or in a light sensor, where individual waves lose their identity. Trying to explain this in terms of photons is possible but will really make your brain hurt (my earlier point).

    There are relatively rare circumstances where two EM waves interact in a non-linear medium and they actually affect each other. There are transparent solids in which laser beams interact and also the Ionosphere will also produce interactions between two radio waves. A very powerful transmission can 'impress itself' (cross modulation) onto a weaker signal from a totally different direction and with a totally different frequency and cause interference.
  7. Dec 5, 2012 #6
    Because most sources of light are incoherent. Here are two examples of light sources: A laser and an incandescent light bulb. An incandescent light bulb gives off incoherent light. A laser beam gives off coherent light.

    The words coherent and incoherent refer to two limits. An incoherent light beam is a superposition of a large number of light modes with different frequencies and wavelengths, with each mode having a different phase. A coherent light beam has a small number of modes with the same phase.

    As a rule of thumb, diffraction patterns are easily seen only with coherent light. Diffraction patterns tend to be stable with coherent light. Diffraction patterns aren't very stable with incoherent light sources.

    Suppose one were to do a two slit experiment first using a laser and then using an incandescent bulb. The slits are placed a few millimeters apart, but less than what is called the coherence length of the laser. The interference pattern from the laser would be very stable over minutes of time. The interference pattern from the incandescent bulb would stay in one position less than a picosecond (10^-12 s). The diffraction pattern from the light bulb would shake around too fast to detect by conventional means.

    Note this is a bit of an oversimplification. There are grades of coherence which I can't discuss without mathematics. There are ways to make some of the light from an incandescent bulb coherent (e.g., clever use of apertures). Further, some of the light from the laser is incoherent.

    It doesn't have to be intense light. I have been in rooms illuminated by laser light. It is very hard to see in such rooms because there are so many diffraction patterns. Practically every dust grain in the room generates a separate diffraction pattern. The name of this type of illumination is speckle. When laser light is used for illumination, the diffraction patterns are more visible than the images of the illuminated objects.

    If a region is illuminated only with coherent light, the speckle pattern is superimposed on the images. The speckle pattern is confusing. The more incoherent light, the easier it is to see the images.

    Coherent light is generated under natural conditions. Our eyes have evolved to process images with incoherent light. However, there are ways to image objects using coherent light. Holography is a means of imaging objects with coherent light. However, it is totally different from the ways of imaging objects with incoherent light.

    The particle nature of light has nothing to do with speckle. Images and diffraction patterns are both easiest to see when the illumination is intense. When the illumination is intense, the particle nature of light has very small effect on the optics.

    Diffraction patterns and images can both be described by classical optics, with no quantum mechanics. Speckle is a classical phenomenon, however weird it may appear. Not everything strange comes from quantum mechanics. There is plenty of weirdness left over in classical optics.

    Lasers are not good illumination sources under normal conditions precisely because the diffraction patterns are so overpowering. If you ever do a holography experiment, then you may find yourself in a place illuminated only with laser light. You will have no problem seeing diffraction patterns. Everywhere!
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