Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Predicting angle of deflection of light

  1. May 2, 2015 #1
    I was recently reading relativity the general and special theory by Albert Einstein. When I came across the equation to calculate the angle of deflection for light as it approaches a massive body. I was just wondering if someone could explain why this works and also what "1.7 seconds of arc" means, as it is part of the equation.
  2. jcsd
  3. May 2, 2015 #2


    User Avatar
    2017 Award

    Staff: Mentor

    The theory was made to match observations. What exactly do you mean with "this"?

    Small angles are described with minutes and seconds of arc. 1 degree = 60 minutes = 3600 seconds.
  4. May 2, 2015 #3
    Oh what I mean is what do the parts of the equation mean.
  5. May 2, 2015 #4
    Oh yeah thanks, so is this what used to predict the angle of deflection?
  6. May 2, 2015 #5


    User Avatar
    2017 Award

    Staff: Mentor

    That depends on the equations, there are different ways to derive that result and I don't know which set of equations you are looking at. I also don't know how much you know about general relativity.
    This is used to quote the result. It is just a unit, like meters for length, seconds for time or degrees for angles.
    Instead of 1.7 seconds of arc, you can also say "an angle of 1.7/3600 degrees".
  7. May 2, 2015 #6
    I'm talking about a=1.7 seconds of arc over delta
  8. May 2, 2015 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The formula you are talking about comes from the formula

    [tex] \theta = \frac{4GM}{rc^2}[/tex]

    Where r is the distance from The center of M that the light passes. The answer will be in radians

    Basically if you use the mass of the Sun for M, set delta equal to 1 solar radius for r, and convert it to give an answer in second of arc rather than radians, you get

    [tex]\theta = \frac{1.7}{\Delta}[/tex] seconds of arc.

    For the angle of deflection of light passing at a distance of delta from the center of the Sun where delta is measured in sun radii.
  9. May 3, 2015 #8
    Ok yeah that makes sense, thanks for the answer.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook