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How come I can see the entire face of a distant object.

  1. Sep 1, 2011 #1
    Hi there. This is my firs tpost so go easy on me. My question is how come we can see the entire face of a distant object. Let me take the sun as an example. When I look at it, i pressume what i can see is the entire face of it that is pointed towards me, but what I dont understand is how. As the light that is eminating from the face i can see also is reaching to my left, to my right above and below me. The way I am picturing it in my mind is if a pebble is dropped in a pool of water, the further the ripple goes, the wider it gets. Now if I were to place a stake in the water 100m away, the stick would only receive a proportion of the ripple that is as wide of the stick. So in effect a small section of the orignal wave. I assume that the same is also true for light leaving the sun? So how come I can see the entire face?
     
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  3. Sep 1, 2011 #2

    tiny-tim

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    Welcome to PF!

    Hi Jaxamercy! Welcome to PF! :wink:

    Yes, most of the light from any part of the face is missing you.

    But some light from all of the face is reaching you, so you see the whole face.
    :smile:
     
  4. Sep 1, 2011 #3
    Thanks tinytim

    Why is this though. Becasue all of the light reaches me at the same time, so it must have been emitted at the same time. Therefore it cant because there are indivdual source of light on the face? So in my "stake analogy" the part of the ripple that reaches the stake contains all of the 'information' from the point of origin? I guess a way of showing what I dont understand is as follows:

    When the pebble hits the water the ripple is close together:

    ABCDEFG (This is my representation just after the pebble strikes)
    ________ (This is the width of the stake at close range)

    Here the stake recieves all of the information

    When it reaches the stake at 100m, I imagine it is like

    A B C D E F G (The ripple grows with distance)

    ________ (The stake width it the same)


    Here it only recieves part of the information
     
  5. Sep 1, 2011 #4

    Ryan_m_b

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    Imagine a sphere covered with dots, if you look at it you can see all the dots on one face. Why is this? It is because those dots are not reflecting/admitting light in one direction but in all directions. Give me two seconds and I'll attach a picture to illustrate...

    EDIT: This is a horrible picture (I've had to do it quickly on paint) but I hope it helps you understand. There are three points (blue, red and green) on a black sphere. They are all radiating light in all directions, the lines represent this emission. The purple star can observe all of them.

    DOUBLE EDIT: Here's a second picture (Spots on a Sphere 2) which illustrates how I think you are seeing this.
     

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    Last edited: Sep 1, 2011
  6. Sep 1, 2011 #5

    tiny-tim

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    No …

    if I shout at you from a distance, you miss most of the sound, but you still get the whole message …

    no information is lost.​
     
  7. Sep 1, 2011 #6
    Thanks Ryan and Tiny-tim

    Sorry for being a bit thick about this! In the example of shouting. If you were to shout "Hello" I can't get my head around why i don't just hear "ell" as the 'H' and 'O' fly past my ears. I know this doesn't happen but don't know why?
     
  8. Sep 1, 2011 #7

    Ryan_m_b

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    I've just added two pictures to my post above, take a look and see if that helps.
     
  9. Sep 1, 2011 #8
    Hi Ryan

    Thanks for the pics it does make it more clear. So in your example, I don't see all of any of the dots, but the becasue there is more than one, I get the entire message? So therefore I shouldn't think of the sun in this example as a single source of light?
     
    Last edited: Sep 1, 2011
  10. Sep 1, 2011 #9

    Ryan_m_b

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    In the same way as if you look at a book from the side you don't see the front yes.
    No don't think of it as a single source, think of it as broken down into loads of dots on the surface because every single point on the surface is radiating light in every possible direction.

    So rather like in the second picture where every dot shines only in one direction every dot is radiating in loads of directions. Here's another picture to illustrate, every point on that face can radiate light that will be caught by the observer, the red lines indicate what the observer can see.
     

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  11. Sep 1, 2011 #10
    That is not correct. The sun has been there for a while, so you are seeing light that has been emitted at different times from different points. At any given instant, the light you are seeing from the center is "younger" than the light from the edges, since the edges are farther away from you.

    Go back to your example of the pond and the ripples. You are correct that if you are a cork bobbing in the water 100m away, you only get a tiny fraction of the energy from the pebble.

    But suppose a second pebble is dropped near the first. Isn't it true that it would send out its own ripples, and that you would also experience them? And the same would go for five, 100, or a million pebbles. All of them would produce waves, and all of the waves would eventually reach you, although it would be a nightmare to figure out the resulting interference pattern.

    If there were a million steady streams of pebbles dropping into the water (assume a VERY deep lake), then before too long, at any given instant, you would be experiencing the effects of waves from all of the splash points -- again, with the waves from those splash points a bit closer to you being "younger" than those from farther away.
     
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