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Thread: My optical problem

  1. #1 My optical problem 
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    Hi,
    Today I was looking in the rear view mirror and noticed and interesting phenomenon.

    I wear prescription glasses because I am short sighted and suffer from astigmatism (I don’t remember exactly what my eyesight is but I know that it’s not too bad.).

    However I noticed that when looking into my rear view mirror with my glasses off, an image that was about 20 meters away looked the same as what I would expect an image to look like if I was directly looking ahead of me without the mirror – blurry and unable to make out details.

    The contradiction I found here was when looking directly ahead I understand that the light is reflected off the object and into my eye; therefore due to my poor vision an object that is x amount of meters away from me will be x blurry/difficult to see.

    What I don’t understand is why when looking at an image that is barely cm’s away from me (since I was looking at the object through the mirror not directly at it) the image still looks as though I were looking directly at the object.

    Logically (to me anyway) the “image” I am looking at is only cm’s away. And the mirror reflects exactly what light gets reflected into it. So then why do I still have difficulty seeing the image?

    I am understand that some part of this will be the fact that I am not only short sighted but suffer from astigmatism but the problem is that if I am looking at an object that is only cm’s away it looks perfectly clear to me.

    Sorry if I wasn’t too clear but this is the best I can explain it.


    "THE ULTIMATE MEASURE OF A MAN IS NOT WHERE HE STANDS IN MOMENTS OF COMFORT
    AND CONVENIENCE, BUT WHERE HE STANDS AT TIMES OF CHALLENGE AND CONTROVERSY."

    Dr. Martin Luther King Jr
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  3. #2  
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    Simply, A mirror simply uniformly reflects all the light, what you seem to be thinking of is a 'screen' just a few centimetres away.

    You can draw the light path(s) from an object, via a mirror to your eyes on paper.


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  4. #3  
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    But that’s kinda the point isnt it? I mean the light is no longer being reflected meters away but only cm's and as you said it should be reflecting it uniformly.
    "THE ULTIMATE MEASURE OF A MAN IS NOT WHERE HE STANDS IN MOMENTS OF COMFORT
    AND CONVENIENCE, BUT WHERE HE STANDS AT TIMES OF CHALLENGE AND CONTROVERSY."

    Dr. Martin Luther King Jr
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  5. #4  
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    Maybe I explained it poorly, have a look through wiki at lens etc.

    I can't easliy explain it without diagrams.
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  6. #5  
    Forum Senior anand_kapadia's Avatar
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    Quote Originally Posted by Megabrain
    Maybe I explained it poorly, have a look through wiki at lens etc.

    I can't easliy explain it without diagrams.
    OI think he is talking about mirror not lens.
    A rear view mirror is a convex mirror.
    The image formed by it is always small.
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  7. #6  
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    If you are talking about optics, then studying mirrors on their own is pointless this thread IMHO is about the interaction of light with the human eye, it requires a knowledge of how a lens focuses light.
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  8. #7  
    Forum Senior anand_kapadia's Avatar
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    Hey no guess what you tried to say.
    I think in your studying syllabe the lens and mirror would have a same topic and you mean to say the same.
    In our syllabus they are distingiushed.
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  9. #8  
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    A lens and a mirror are distinctly different, the original poster indicated a mirror (the rear view one in his/her car), in order to understand why they are seeing things distorted through short-sightedness and astigmatism reflected in a mirror, you need to understand how both work.
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  10. #9  
    Forum Sophomore Kabooom's Avatar
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    I'm not quite sure what MB is saying because I don't really care how mirrors work, but to anand_kapadia: He's not saying that mirrors and lenses are the same, but that to understand how your eyes focus you need to understand how a lens works since the eye uses a lens.

    (or at least that's what I got from it...)
    WHAT?!
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  11. #10  
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    maximg, I had to read your confusing question twice, but I think I now understand what you're asking: You expect to see everything in the mirror clearly, even though you're short-sighted, because the mirror is right in front of your eyes.

    That would really be quite a revolutionary discovery, if it worked, but it doesn't work that way. Your eyes are not focusing on the mirror surface, but they are focusing on whatever the mirror reflects, which is far away. The distance between the mirror and your eye is almost irrelevant.

    Consider this: You are looking at another car, located 20 meters behind your head, through a mirror which is 30 cm in front of you. What length does light travel from that car to your eyes? I travels about 20.3 meters to the mirror, is reflected there, and then travels 30 cm back into your eyes, to a total of 20.6 meters. This is the distance to the car, that your eyes will perceive. Of course, everything will be blurred unless you wear your glasses! In fact, you would see a little more clearly if you turned your head and looked at the car directly (about 20 m distance).

    On the other hand, if you were not looking at the reflected image, but focusing on that scratch on your mirror surface, your would indeed see it very clearly, because that's really just 30 cm away. Clear? :-D
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