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Basic Optics

  1. Jan 22, 2014 #1
    Very basic optics questions:
    1. So light is a type of radiation, due to which we see objects. Some objects, like a candle, are luminous and we can see them without another light source. Some other objects are non-luminous, and we need a light source to have light reflected upon them. How does this work? Does the light source release light rays which are reflected from the object and enter our eyes? When they enter our eyes, how do we see the object and know where it is? Is the object located from where the rays originate/intersect?

    2. Can someone explain real images as to what it means with respect to being displayed on a paper screen?

    3. Are real/virtual images distinguished in the sense that if the extrapolated rays behind mirror intersect, then it's virtual and if they intersect in front of the mirror it's real? Why is that?

    5. In real/virtual images, why are images located where the reflected rays intersect/originate?

    Not homework, just need my basic concepts clarified. Thanks.
  2. jcsd
  3. Jan 22, 2014 #2


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    Staff: Mentor

    Right. Some tiny fraction of the light reaches your eyes, most of it hits something else.

    Our eyes can detect the direction of the incoming light (light from different directions hits different cells in the eye). Our brain then estimates its distance with many different methods:
    1) for objects nearby, our two eyes receive light from this object in slightly different directions, and this difference depends on the distance
    2) if we are moving, the direction varies with time, and the same concept as (1) applies
    3) our brain knows typical sizes of objects and can compare this with their apparent size
    4) if one object is in front of another, it is closer to us
    5) probably some more I forgot

    "All" light emitted from a single point of the displayed object hits the same position of the screen - some part of it gets reflected, and we can see it on the paper as a result.

    In a real image, the light is really there - you can put a paper there for example. Virtual images are just a mathematical tool.

    If you put a sheet of paper somewhere else, everything overlaps and you don't get an image.
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