Why light from sun takes 8 minutes ?

  1. Why is it said that light takes 8 minutes to reach earth when light from sun is already present in the solar system , what is meant exactly by the statement ?
     
  2. jcsd
  3. D H

    Staff: Mentor

    It means that a photon emited by the Sun that hits the Earth at some point in time left the Sun 8 minutes prior to the time it hit.
     
  4. if you're at a concert and standing about 330 meters away from the speakers then the sound you are hearing is about 1 second old. this is because sound travels in air at about 330 meters every second.
    its a simular situation with light, except it travels much much faster so that normally on earth we dont notice any delay, it travels 300,000 kilometers per second.
    the sun is aprox 150 million kilometers away, so it takes light about 500 seconds to get here or 8 minutes.
    essentially when you look at the sun, you see a picture of the sun that is 8 minutes old.
    if the sun suddenly stopped shining, you would not know for 8 minutes.
    if it then began to shine again, we would not recieve the light for 8 minutes.
     
  5. but sun doesn't stop shining , the photons are already present around the earth , sun is burning whether the side on which of earth i live faces the sun or not , its already there yet we say it takes 8 minutes
     
  6. Hurkyl

    Hurkyl 16,089
    Staff Emeritus
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    Gold Member

    Right; but only becuase the sun was burning 8 minutes ago.
     
  7. Photons are like little bullets, so just try to picture someone sitting on the surface of the sun firing off photon bullets at the earth. The sun is ~92,000,000 miles from earth, and the bullets (photons) travel at 186,000 miles per second, so it takes them about 8 minutes to get here from the surface of the sun. Just like if you were to fire a bb gun that shoots bbs at 1000 ft/sec, it would take them 5 sec to hit a target at 5000 ft.
     
  8. tiny-tim

    tiny-tim 26,044
    Science Advisor
    Homework Helper

    There is nothing new under the Sun …

    The river is always before us, but the water comes slowly from the mountain.

    The present is but a record of the past.

    All, all is vanity.
     
  9. I'm glad you stuck with your question. There's a depth to these types of questions that makes it sometimes difficult to know where to start the answer. Eventually somebody will click with your question.
    Let me ask back:
    Why is it said that rain takes about 10 seconds to reach the earth when rain from a cloud is already present under the cloud?
    [...]but the cloud doesn't stop raining, the rain drops are already present at the earth under the cloud, [...] rain is already there yet we say it takes 10 seconds.

    You might notice a problem with my question: "Rain drops are already present at the earth". In this case, you understand that only some of the rain drops are present at the earth. Looking up you know that many others are still on their way following the drops that hit the earth. If we put green dye in the cloud it would take 10 seconds for the green stuff to reach the earth.
    As for the sun, although we are drenched in sunlight, only some of it is at the earth. Looking up, fresh sunlight is still coming to us following the light that hit the earth. If we added more green light to the sun it would take 8 minutes for the green stuff to reach the earth.
    Back to the cloud: If a hole formed in the cloud, allowing light to come through, the light would be much faster than the rain, but it would still take the smallest fraction of a second to reach the earth.

    I think I've noticed the "already present everywhere" statement elsewhere. This seems to be the heart of the confusion. May I recommend reading: http://www.teachersdomain.org/resources/lsps07/sci/phys/energy/lightspeed/index.html. This particular concept of finite speed of light isn't mystical.
     
    Last edited: May 24, 2008
  10. Redbelly98

    Redbelly98 12,037
    Staff Emeritus
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    The amount of light emitted by the sun is not constant, it changes continuously. For example, when a sunspot on the surface of the rotating sun rotates into view (as seen from Earth), the total light output drops by a small amount.

    On Earth we don't observe the sunspot, or the drop in light output, until 8 minutes later.
     
  11. think of it this way... if the sun suddenly exploded, we wouldnt know about it until 8 minutes later.
     
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