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I Why do convex lenses magnify, and why don't concave lenses?

  1. Aug 12, 2016 #1
    So knowing that convex lenses causes light to converge and concave lenses causes then to diverge, why does converging light create a larger image than diverging light?

    Magnification means we want to see more of a certain part of an object, meaning we want to enlarge that section and have it cover up as much of our image sensor (camera, eyes) as possible, right?

    So if there was an image sensor, then wouldn't the diverging light from a concave lens would cover more area on the image sensor than the converging light of the convex lens and thus create magnification?
     
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  3. Aug 12, 2016 #2
    In physics magnification is a well defined quantity. It is size of image/size of object. The size can be a linear size or an angular size.
    It does not simply relate to how much of an object we want to see.
    coverging and diverging lenses can both produce magnification...depends on the application.
     
  4. Aug 12, 2016 #3

    sophiecentaur

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    This link (http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/lensdet.html) and other pages in hyper physics, may help to give you some basics of the way the optics of lenses works.
    Whether a lens produces a larger or smaller image, depends on several factors and an image is where Rays appear to or actually do come from points in space.
    Your ideas don't fit in with what hyperphysics ( or any basic text book) will tell you. Read around the topic more.
     
  5. Aug 12, 2016 #4
    It would cover more area but the rays would not come to a focus, so there is no image on the sensor. Magnification is (size of image)/(size of object) and for the concave lens illuminating a sensor there is no image.
     
  6. Aug 13, 2016 #5
    Ok. But how come the diverged light rays that arise after the focal point from a convex lens are not blurry, and the diverged light rays that arise from concave lenses are blurry?
    Because in shortsightedness the light focuses in front of the fovea, and after the focal point, the light spreads out and you see a blurry image. Yet diverged light rays from convex lenses are not blurry?

    As it's described here http://www.olympusmicro.com/primer/anatomy/magnification.html
     
    Last edited: Aug 13, 2016
  7. Aug 13, 2016 #6

    Drakkith

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    The images in your link are NOT how the focusing of light by a lens (or your eye) works. Standby. I'll find an accurate one.
     
  8. Aug 13, 2016 #7

    Drakkith

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    Alright. Below is an diagram of a basic microscope. The light from a single point on the object (in this case the tip of the arrow and only the tip of the arrow) is shown. That last bit is important. Light emitted/reflected from another point on the object, for example the middle of the arrow, would undergo a different path through the optical system but would still emerge from the eyepiece as a near-parallel bundle of rays like the cone of light shown. Note that the placement of the eye is actually incorrect here. The pupil would be placed on the right side of the eyepiece and would be much larger to catch all of the bundles of light emerging from the eyepiece, not just the single bundle shown.

    Once the bundle of rays passes into the eye, the cornea and lens focuses the bundle down to a point on the retina. If the bundle is not focused to a point then you wind up with a blurry image.

    microscope.jpg
     
  9. Aug 14, 2016 #8
    I think this helped a bit.

    So magnification is going to be the image created at the focal point, but it's possible to create a lens that bends light at super high angles so that maybe only 1 inch away from the lens, you can get a very high magnification? And likewise, it is possible to create a lens that bends light at very low angles to create an image that is same height as the object at a focal point very far away from the lens?

    But I now have 3 questions:
    1) I now understand that the reason why Image 1 is larger than Object is because the largest Y coordinate where the focal point is results in a larger height of Image 1 than the height of Object. But why does the eye see a much larger image (image 2), than image 1? Can you draw a diagram where the eyes are replaced with lenses?

    2) Why does light have to be focused to a point to be clear? What is it about diverging light that makes an image blurry? Like in this image, point 1 will see reflection from point A, point 2 will see reflection from point B, and point 3 will see reflection from point C. So why does this not result in a focused image?

    http://imgur.com/a/w9Eup

    3) If light is needed has to converge to a point to be focused, how come digital cameras can take clear pictures if the camera sensors are rectangles and not points?
     
    Last edited: Aug 14, 2016
  10. Aug 14, 2016 #9

    Drakkith

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    I'm afraid I can't draw a diagram, nor can I find a good image. The reason the eye sees a larger image is because the rays enter the eye at a larger angle and get focused down onto the retina further from the center than they otherwise would be. Unfortunately I don't think I can describe it much better than that.

    The camera sensor is composed of points in a 2d plane. The light is focused down onto these points.
     
  11. Aug 14, 2016 #10

    sophiecentaur

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    To get a high magnification with a single lens (magnifying glass) you have to be able to focus very close. Your 'near point' is personal to your particular eye and it gets harder and harder to see as you adjust the object and lens position. Try it out and see what I mean.
    The earliest microscopes used a small and very fat piece of glass as a single lens, set in a hole in a metal sheet. Very hard work to use. Life got easier when they invented a two lens microscope which puts its image at a distance you can see without eye ache.
     
  12. Aug 14, 2016 #11

    Drakkith

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    Personally I like the "image at infinity" setting. :wink:
     
  13. Aug 14, 2016 #12
    light rays that arise after the focal point from a convex lens: These are diverging from the focal point, just as the light diverges from any object you look at without a lens. Your eyes converge these rays onto the retina.

    the diverged light rays that arise from concave lenses are blurry: Not if viewed with your not-(short or long sighted) eye. The eyes again will focus the rays onto the retina and the image appears to come from a point beyond the lens. Some people wear concave lenses and the image is not blurry.

    in shortsightedness the light focuses in front of the fovea, and after the focal point, the light spreads out and you see a blurry image: now we are talking about a situation where the eye lens cannot focus the rays onto the retina, hence a blurry image.
     
  14. Aug 14, 2016 #13
    Visual instruments such as magnifiers and microscopes work by increasing the angle that the final image makes at the eye compared to the angle that the original object makes at the eye - they provide angular magnification. In these two instruments, the final image is virtual i.e. the rays diverge as they leave the instrument, as in Drakkith's diagram. (I would have drawn that with the eye centered on the optical axis).
     
  15. Aug 17, 2016 #14
    Do you know Snell's law? Consider that air has an index of refraction close to 1 and lenses are made with material with index of refraction greater than 1.
     
  16. Aug 18, 2016 #15
    this may not give greatest angular magnification.
     
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