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Homework Help: Does light from a point source spread out?

  1. Apr 27, 2006 #1
    I have been looking into the strenth of an infrared light source at different distances from a point source. My teacher said that the light would not spread out but when I looked on the internet i found something that said the strength was inversly proportional to square of the distance from source, which is correct and why?
  2. jcsd
  3. Apr 27, 2006 #2


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    You are right. Maybe your teacher talking about a laser? It doesn't follow inverse square, but it does spread out.
  4. Apr 27, 2006 #3


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    When talking about a point source in optics it normally means that the physical size of the source of emission is very small. That means that all of the rays coming from the source originates at a "point" in space. It also means that the rays are diverging from the point source in "all" directions. I put the all in inverted commas since this is not possible in practice since the rays will normally fill only a small cone centered on the source and extending away from it. The amount of radiation from a point source will decrease according to the inverse square law with increasing distance.

    Other sources of emission like lasers (and infrared lasers) also emit rays from a small region in space, but the rays coming from them are travelling parallel to each other, we say it is a collimated beam. In this case the amount of light will be constant irrespective at what distance you are from it (if you stay in line with the beam!).

    Plasma lasers create their light in a tube. The rays bounce back and forth between the endpoints of the tube. The rays that are not travelling along the tube exit it along the side, while the rays travelling along the tube will eventually exit it at one of its ends thereby producing a collimated beam.

    Solid state lasers emit their rays from a small region in space - the junction - the region where the semiconductor materials are joined. This will act like a point source, but normally a lens is used to collimate the rays by using a convex lens with a short focal length. The lens is positioned so that its focal point is located at the point source. This will cause the rays to be parallel on the other side of the lens. The lens need to be of such material so that it is transparent to the rays though!
    http://www.repairfaq.org/sam/laserdio.htm" [Broken]
    Last edited by a moderator: May 2, 2017
  5. Apr 27, 2006 #4
    Do you no if the strength of the light follows any pattern or formula?
  6. Apr 27, 2006 #5
    What do you mean by "strength of the light" ? Are you referring to intensity ?


    ps : your teacher was talking about laser light indeed.
  7. Apr 27, 2006 #6


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    There are no conformed standards as far as the production of these devices are concerned. If you cannot get hold of the data sheet on the particular product it is best to investigate it experimentally, although finding a device that can actually measure the infrared light and setting up conditions for such a procedure might prove quite challenging! So I would opt for hunting for the data sheet if I were you. Look at the link that I inserted at the end of my post. It may answer some of your questions. Why are you interested in this property of your source?
  8. Apr 27, 2006 #7
    Part of a courseworky thing
  9. Apr 27, 2006 #8


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    So they expect you to actually measure it? What do you know about the source? Producer? Product number?
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