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2 Qn regarding Optics and Pressure

  1. Sep 7, 2006 #1
    helo guys
    i hav 2 qn to ask here
    from wat i read from web and books, a virtual image is a image which form behind the mirror, jus because of light cant pass thru the mirror but our brain think it as a image behind mirror
    so my qn is where does the image actually form?

    second qn, a syphon
    y the pressure at upper part can be neglected?
    then, the mass at the upper reservoir is so much bigger than the column of water which go down to lower reservoir, y the water tend to go down thru the column of water which go down to the lower reservoir?

    thanks for replying and viewing!

    Attached Files:

  2. jcsd
  3. Sep 7, 2006 #2
    For your first question, think about what your eyes are actually doing. They are recording light hitting them. The image is forming in your eyes.
  4. Sep 7, 2006 #3


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    For your second question, Google on siphon and see if you can dig up an article or two about how they work. Once you understand the principle behind the siphon you will understand whether the volume and mass of liquid in the upper reservoir and the the pressure over these open containers have any bearing on its operation.
  5. Sep 19, 2006 #4
    i think 1st qn i hav no prob d, thanks
    for second qn, i'd search thru google, but couldnt find an article tat explain in details
    wat i interest to kno is the "h" onli, y it onli nid to draw til the surface of water but not the whole column of liquid?
  6. Sep 19, 2006 #5


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    A real image is formed when the light rays come to an actual focus. This sort of image can be seen on a screen by inserting a screen at the point where the light rays are focussed. The screen will then reflect the light rays and it will seem that the picture of the object is actually located on the screen. This is because the optics system maps each point on the real image to a point in the focal plane. That is the picture is reformed with its original light rays.

    With a virtual image we find that the light rays do not actually come to a focus and the image do not reform. In this case the rays diverge in stead of converging.

    But coming back to the case of the real image. The light rays actually continue beyond the point where they converged (if the screen is not inserted). This means from that point onwards the rays actually diverge. If one were to look at this region where the rays converges with one's eye one would see the actual picture forming in this plane of convergence. So looking up against diverging light rays causes one to "see" the actual image. The same happens with a virtual image, but in this case the light rays did not actually converge at any stage.

    It is not clear to me what your question is regarding your second problem. Do you not understand how a siphon work?
  7. Sep 19, 2006 #6
    emm if the screen is placed exactly at the point where the point of intersection, the image is sharp, if the screen is place backward or forward the image become blur, can anyone explain tis?
    second qn is...emm wel i understand tat the column of water outside the reservoir is longer hence gravity force pull down there and create and partial vacumn which cause the liquid in reservoir to flow out, and the factors of cohesive force 2
    but wat i cannot understand is y the effective pressure using the h in the diagram, the h onli up til the surface of water but not the whole column?

    sry but my brain really limited...sweat
  8. Sep 19, 2006 #7


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    It is like drawing an image with little dots. If the brush makes larger dots then the image will not be so sharp, that is detail will be lost. So each point of light is not focussed to a sharp point. The convergent rays form a larger "point" when the screen is not in the focal plane. These convergent rays form a cone of light for each point in the image. The large side of the cone is the surface of the lens. The closed point of the cone is where the rays come to a focus. The cone of rays continue on the other side of the convergent point. So if you are not at the focus the point is more of a circle.

    See this excellent site for an explanation of the working of a siphon:
    http://www.pump-flo.com/company/pumpcenter/siphon.pdf" [Broken]
    Last edited by a moderator: May 2, 2017
  9. Sep 20, 2006 #8
    i checked the site, it provide many explaination, but the sentence "the difference in height between point 2 and 6 provide the energy" wifout any explaination make me disappointed once again...i nid some numerical explanation, but i cant done it myself as i hav tried for a long time, sigh
  10. Sep 25, 2006 #9
    anyone can explain to me?...
  11. Sep 25, 2006 #10


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    Last edited by a moderator: Apr 22, 2017
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