1. Limited time only! Sign up for a free 30min personal 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!

Formula for a torch

  1. Jun 17, 2010 #1
    Hi i'm new here and I'd like some help with something that's been niggling me for the past few days. I've yet to study Maths or Physics at A Level (UK), so this should be easy for some of you.

    If I were to shine a torch that shone a circular, even spreading of light, what would be the relationship between the angle of the light coming from the torch, the distance from a wall, and the brightness on the wall?

    Here's what i've got: (this is assuming there isn't a property of light that I don't know about)

    D=Distance from wall
    X=Angle, As in Shining at 90 degrees would actually shine 90 up and 90 down


    D*TanX=Radius, so pi*r*r=a


    However it just doesn't seem right. Is this right or wrong?
    Last edited by a moderator: Apr 25, 2017
  2. jcsd
  3. Jun 17, 2010 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

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