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Why do things far away seem smaller?

  1. Feb 9, 2015 #1
    i know it sounds like a very stupid question, stated that it's very much given that the further away the observer is from the source of light, the smaller they seem...

    BUT WHY???
    i just can't wrap my head around the fact....
    can someone explain to me, why it seems like that, and do you think this applies to every time and dimension?(even before bing bang, since we are talking such fundamental things)
     
  2. jcsd
  3. Feb 9, 2015 #2

    Simon Bridge

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    The light from the object ends up taking less space on your retina - you can verify that with ray diagrams.
    It's the same reason distant objects make a small image in a pinhole camera.
    If you still have trouble - try tracing the light rays from an object using pins.

    As to "why" ... light rays behave that way because of the way the Universe works.
     
  4. Feb 9, 2015 #3

    DaveC426913

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    Consider what you'd see if distant objects were not seen as smaller.

    Looking at a 1 foot cube 10 feet away, it has a front face that's a square and a rear face that's a square.

    Draw straight lines (rays of light) from the four corners of the front square to your eye, and from the four rear corners to your eye.

    If the rear square did not subtend a smaller angle than the front square, you would be able to see around the front of the box at the sides!

    cube-png.78835.png
     

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  5. Feb 10, 2015 #4

    A.T.

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    The angular size depends on distance:

    The brain corrects for this, if it knows the object is far away, so we don’t look at distant people, and wonder if they are actually tiny dwarfs. This correction can also demonstrated in the below picture, where the left guy seems taller because he is assumed to be further away, despite having the same angular size as the right guy:

    Perspective-train-station-optical-illusion.jpg

    The reason why the correction doesn't work so well for a distant light source, like a star, is because you have little visual cues on how far it actually is. With just a black background, a torch 1km away, a plane light 10km and a star lightyears away look pretty much the same.
     
    Last edited by a moderator: Apr 15, 2017
  6. Feb 10, 2015 #5

    Andy Resnick

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    At the risk of creating confusion:

    Entocentric lenses are the most common form; the entrance pupil is located within the lens and the image magnification varies with object distance (perspective distortion).
    Telecentric lenses place the entrance pupil at infinity, and show no perspective distortion- the magnification is independent of object distance.
    Pericentric or Hypercentric lenses place the entrance pupil in front of the lens and can image all the sides of objects (except the rear...)

    http://www.opto-engineering.com/resources/telecentric-lenses-tutorial
    http://www.opto-engineering.com/resources/peripheric-optics-tutorial
     
  7. Feb 10, 2015 #6
    i thought that at first, that's the logical answer, however, my question arises after that thought

    if further things only look smaller because our eyes, then how is so many stars filling our sky (i mean how are we obseving so much of them, cuz as far as i know radio receptors has no lenses...), and waves radiate
     
  8. Feb 10, 2015 #7

    DaveC426913

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    No. Farther things look smaller because they physically subtend a smaller angle in our eye. Our brain interprets smaller angle as either farther away or smaller, depending on other visual cues.

    But the smaller angle is still there, even if our eyes aren't. For example, a planet at Pluto's distance will receive much less light from the sun than a planet at Mercury's distance, because Pluto subtends a much much smaller angle of the celestial sphere (from the Sun's position) than does Mercury.

    angles.png


    Not sure what you're getting at. We see stars in the visible spectrum of light.
    Radio telescopes see them in the radio spectrum.

    Regardless, both visible light and radio light travel in straight lines, so the triangles, as in the above diagram, are still there.
     
    Last edited: Feb 10, 2015
  9. Feb 10, 2015 #8

    Drakkith

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    Radio telescopes are reflective telescopes. They usually have a main dish shaped in such a way as to focus the radio waves just like a lens does to light waves. An antenna located at the focal point of the dish detects the waves and lets us 'see' them, much like how a CCD of CMOS sensor detects the light at the focal point of a camera. Since all telescopes see only a small portion of the sky, we have to move the radio telescope around and point to different objects to see them.

    One key thing to understand is that you can have an antenna without a dish. You just won't be able to tell which direction your signal is coming from, block out signals you don't want, or form images. That's the purpose of the main dish. It gathers radio waves coming from one direction and focuses them onto a spot. Radio waves coming from he another direction are either focused to a different spot (if the location is on the correct side of the dish) or are blocked completely.

    Similarly, if you took the imaging sensor in your camera and placed it outside under the sun without a lens, the light from the sun would be spread out all across the sensor and you wouldn't have a recognizable image. The purpose of the lens is to make sure that light coming from a single direction is focused onto only one spot, not spread out over the whole sensor. So if you take a picture of a bird on a limb, the lens makes sure that the light coming from the bird ends up on only some of the sensor's pixels, while the light from the tree limb ends up on different pixels, etc. Each pixel has no idea where the light came from. It just says "Hey, I measured this much light". It's only when we put together the output from many different pixels that we get an image. The rods and cones in your eye function just like the pixels in a camera's sensor.

    Yes, light and radio signals are indeed a wave. As this wave propagates away from the source the wavefront spreads out as it goes. A lens (or reflecting telescope) looking at the source will have part of the wavefront enter its aperture. It is this small section of the wavefront that is focused down to a spot. The rest of the wavefront simply passes by and keeps going.
     
  10. Feb 10, 2015 #9
    Simply it takes up a smaller percentage of your field of vision. Imagine holding something so close to your eye that it blocks everything else out, as you move it away it will stop blocking so much out and take up a smaller percentage of your field of vision so it will look "smaller"
     
  11. Feb 16, 2015 #10

    Merlin3189

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    The Moon Illusion is an interesting case. If you look at a full moon when it has just risen or is approaching setting, it looks much larger than when it is high in the sky around midnight.
     
  12. Feb 16, 2015 #11

    Drakkith

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    Yes, but that's an optical illusion, not a real physical phenomenon.
     
  13. Feb 16, 2015 #12

    sophiecentaur

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    It brings to mind the Father Ted episode when they are looking at cows in a field from the caravan. Dougal gets a lesson in perspective. Perhaps Father Ted has [not been] screened in the US. A great loss to you all.
     
    Last edited: Feb 16, 2015
  14. Feb 16, 2015 #13

    DaveC426913

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    Autocorrect? :wink:
     
  15. Feb 16, 2015 #14

    DrGreg

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  16. Feb 16, 2015 #15

    DaveC426913

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  17. Feb 17, 2015 #16

    sophiecentaur

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    It really doesn't help that the Moon happens to look the same size as the Sun from here.
     
  18. Feb 17, 2015 #17

    Merlin3189

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    I had assumed the geometric argument was obvious and trivial, so we were on to psychological explanations from post #4.

    Although I do not know the explanation of the Moon Illusion, the link contains descriptions of several proposed explanations of size illusions which I thought might be relevant to your problem.
    I find the Moon Illusion powerful because it persists despite knowing that it is an illusion. Even when one is demonstrating that fact by experiment, the impression still remains.

    I must confess I don't understand the original question, so maybe I've missed a point.
     
  19. Feb 17, 2015 #18

    Drakkith

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    I thought that was the entire question.
     
  20. Feb 17, 2015 #19

    DaveC426913

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    Concur with Drakkith.

    In a sci-fi story I read, one of the things that attracted tourists to Earth from all over the galaxy was that it was unique for its spectacular Ring of Fire eclipses. No other body in the empire had the right combination of moon size, sun-size and distance. Kinda cool to think that, in actual fact, we are special (or at least, part of an elite club) in the galaxy for something.
    And now back to our regularly scheduled program...



    There are lots of thread on PF discussing it. I have my own pet explanation, but let's not derail his thread any more than I just did.
     
    Last edited: Feb 17, 2015
  21. Feb 19, 2015 #20

    sophiecentaur

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    What a pair of spoilsports! This is a topic that could run and run. The clue is in the word "seem". :wink:
     
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