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B Why do eyes only see in straight lines? (Light Physics)

  1. Mar 30, 2016 #1
    Hello! Please help me with this question.

    I'm currently studying Light and Geometric Optics and have become quite confused.
    When looking at diagrams of mirrors, and light relfection into eyes, why is it that eyes only see things when tracing the light in straight lines? For example, if an apple were placed in front of a mirror, light rays would reflect off the apple and hit the mirror, thus reflecting in parallel lines that are percieved in our eyes. However, our eyes trace this light back into the mirror as though it came from the other side of the mirror. Why do our eyes see in straight lines and trace the light back into the mirror?
    And with diffuse reflection, the light is just scattered, yet we can still see the object. So, why is it that we can only see perfect relfected images if the light rays are parallel to one another?
    Please help! :frown:
  2. jcsd
  3. Mar 30, 2016 #2
    You eyes trace the image beyond the mirror because each eye sees the image at a different spot on the mirror surface.
    That a sure-fire cue that the image must be further back.

    Look at your nose in the mirror. Then, without moving, cover one eye and note where the image of your nose appears on the mirror surface. Then do the same with the other eye. You will notice that the two spots on the mirror (left eye and right eye) are about an inch apart.
  4. Mar 30, 2016 #3
    Your question is not so much about physics but neuroscience.

    Normally light travels in straight lines. There are exceptions, but only one is relevant here: reflection. There's also refraction and bending of light by intense gravity, but we can ignore them. This property is roughly equivalent to "light rays travel parallel to each other" although not in detail.

    Eyes - more to the point, the brain's visual processing capabilities - have evolved to know and expect light to travel in (parallel) straight lines. When you look in a mirror, your visual centers didn't evolve for that, so it tells you the object's on the other side, where it appears to be. With higher level intelligence (neocortex) we have no problem figuring out what's going on; but we need to figure it out; it's not automatic.

    Try this: watching your hand in a mirror, touch something you see there. At first it's difficult but you can get good at it.

    Look in a convex or concave (like, a makeup) mirror, or through a lens, to see how strange it looks when the rays aren't parallel.

    If light is very diffused the image is lost. Use ray tracing, you'll find diffused rays go all over the place. But with just a little diffusion it's different. Your brain is very good at automatically interpreting partial information so it can decode images when they're diffused and distorted.

    Here's a puzzle, helps to clarify what a mirror does. Why does a mirror reverse your left and right sides, but not top and bottom? You see yourself standing normally but reversed side-to-side - why? Use what they've taught you about ray-tracing to figure it out.

    Hope this helps.
  5. Mar 30, 2016 #4
    Because light travels in straight lines. In most cases.
  6. Mar 31, 2016 #5
    Thank you so much guys! This does help heaps and makes complete sense too! So all in all, our brain is just hardwired to expect light to travel in straight lines and traces the light back through the mirror because our eyes see two different images and believes that there is a real image behind the mirror. Thank you so much for your time and help!:smile:
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