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Light rays in concave lens,moon and eclipses?

  1. Sep 9, 2009 #1
    Hello everyone,

    Quick questions


    What happens to the light ray that passes through the mid point of the lens(dotted line). Does it go straight or diverge?


    Is moon concave? If the light passes right through the midpoint travel straight wouldn't the umbra have some light in the middle. Why is penumbra a shadow? Why are eclipses rare. It is so easy for moon's shadow to fall on the earth. How is moon big enough to provide light for whole one side of earth.

    Thank you so much!!
  2. jcsd
  3. Sep 9, 2009 #2

    Doc Al

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    What does your question about a diverging lens have to do with the moon and its shadow? (The light ray passes straight through the center of the lens.)
  4. Sep 9, 2009 #3
    Thanks!! So how does light defract from the moon. How is the shadow created. I though surface of moon is concave. Please englighten my poor understanding.
  5. Sep 9, 2009 #4


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    It's not behaving as a lens system as all. The sun is not a point source, the light from one side of the sun is traveling to the opposite side of the Earth (as indicated in the figure) and the partial blocking of the sun's light creates the penumbra. The umbra is where all the light from the sun is being blocked.
  6. Sep 9, 2009 #5
    Thank you blue region has light right (penumbra). So why are eclipses not so common. I mean it is very easy for earth's shadow to fall on the moon or vise versa?
  7. Sep 9, 2009 #6


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    The drawing is not to scale. The distance between the Earth and moon is about 30 times the Earth's diameter, while the drawing shows them to be much closer.

    Image the moon farther off to the right in the drawing, where the umbra is a lot smaller. Then it is easy for the moon to miss the umbra entirely.
  8. Sep 9, 2009 #7
    ok thanks mate. Also quick question as the distance increases why does the shadow lose its intensity allowing more light?
  9. Sep 10, 2009 #8


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    Technically it doesn't lose intensity, but it does get smaller. It's a matter of geometry, and the fact that the sun is larger than the Earth.

    If you're in the penumbra region, you can see part of the sun -- therefore that region is partially illuminated.

    If you're in the umbra, the sun is completely blocked -- therefore the umbra is completely dark (Ignoring the light that is refracted by the Earth's atmosphere into the umbra)
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