1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

any genius answer my question

  1. Aug 17, 2005 #1

    CAn any genius solve this question.I have tried my best but iam unable to get the answer.

    QUESTION 1 ; "Stars are know to flicker.Estimate the number of photons entering the eye of an observer when he looks at the star of first apparent visual magnitude.Such a star produce flux on the surface of the earth of 10 (exp) -6 lumens/m^2 .One lumen is 0.0016 watts.Star Aldebaran is an example . Why do stars flicker,Not the palnets or very little.

    QUESTION 2 :In positron atom both the electron and the positron mive with the center of amss that is sationary .Calculate the ground state energy of the positron atom.
  2. jcsd
  3. Aug 17, 2005 #2
    I think stars flicker more than planets as they are lot further away.The intermediary medium has varied properties so they appear as blinking.
  4. Aug 17, 2005 #3


    User Avatar
    Staff Emeritus
    Science Advisor

    But the "intermediary medium" is, primarily, the atmospher which is just a deep for planets as for stars. The actual reason is that planets have a disk while stars do not. If you look at planets in a telescope, all those that are visible with the naked eye, you will see a disk. If you look at stars in a telescope, no matter how how powerful the telescope, you see a single point. Even with the naked eye, you see a tiny disk, as opposed to a point of light. A disk is not as easily covered or "warped" by a change in the density of the atmosphere as a single point.

    Oh, and to find how many photons enter the eye, you will need to know the size of the pupil as well as the time period involved.
  5. Aug 19, 2005 #4


    User Avatar
    Science Advisor
    Homework Helper

    Stars flicker because of atmospheric effects only. Think about what the sun looks like when seen from the bottom of a pool of water with ripples in the surface. The flicker of stars is similar, but instead of the varying surface of a pool, the reason light gets bent in various directions in the atmosphere is due to the fact that air does not have uniform density.

    Sometimes, if the air is very still, the stars will not flicker.

    The flicker can be described two different ways. What your eye percieves is a change in intensity. But what is really going on is a change in direction. Sometimes the intensity is lower because the light changed direction and missed you.

    Both a star and a planet are "disk" sources of light, but a star is far smaller. A planet is so much larger (in angular area) that there is always some light that makes it to your eye. Another way of putting this is that the amount of change to the direction of light due to flicker is LESS than the angular size of the planet. Thus the planet stays in view and appears stable.

    The flicker effect has nothing to do with the number of photons entering the eye. In both the case of a planet or a first magnitude star, you will find that the number of photons received by the eye is a fairly large number, per second.

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: any genius answer my question