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I Why isn't there light exiting anywhere else?

  1. Apr 28, 2017 #1
    In this video, a laser is being shined through a block of plastic with changing density.

    At time 0:13, light enters the plastic and exits. The light takes the path the way it does because that path provides the least time to arrive at the current exit

    But why doesn't the laser also take another path to arrive at another exit below the current exit? Surely there is ANOTHER path of least time that would allow light to exit at another position?

    I imagine that instead of a laser we use a lightbulb, the whole block of plastic would be illuminated and each possible exit point would have light passing through it. What's up with a laser?
  2. jcsd
  3. Apr 28, 2017 #2
    In the end, you're asking why, when a ray of light enters water, it gets refracted at a certain angle, not at others. That's essentially Snell's Law, so I would recommend reading that.
  4. Apr 29, 2017 #3
    Yes, I know of Snell's Law, but what I'm confused about is why the light doesn't get reflected everywhere to cover all possible points.

    If I shine a regular lightbulb through a piece of plastic, all possible exit points on the other side of the plastic are illuminated. Not so with the laser, as shown in the video.

    The laser can only produce light in one direction?
  5. Apr 29, 2017 #4


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    Staff: Mentor

    The intensity of light from a laser is almost entirely contained within a thin beam than only slowly spreads out as the light travels. Only a very small amount of light emerges at a wide angle from the laser. This is unlike a regular light bulb where light is being emitted in every direction from each point on the surface of the filament (or phosphor coating if we're talking about fluorescent lights).
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