Power of LED and human eye

  • Thread starter prehisto
  • Start date
  • #1
prehisto
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Homework Statement


In ideal conditions human eye can feel something if 100 photons with wavelengh of 5500A fall directly tu human eye in one seconds time.LED(lambda=5500A) is placed 5m away from human eye.


Homework Equations



Calculate the power of LED to achieve that the human eye can see it?

The Attempt at a Solution

 

Answers and Replies

  • #2
voko
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What is your difficulty?
 
  • #3
prehisto
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I even don't know where to start here.
Some kind of idea would be helpfull.
 
  • #4
voko
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Each photon carries energy. How much?
 
  • #5
prehisto
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I know that much
E=hv and v=c/lambda so E=hc/lambda
 
  • #6
voko
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So you should be able to solve the problem.
 
  • #7
Antiphon
1,683
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You also need to know how many photons miss or hit the pupil. Perhaps they all bit because of lenses. Or not.
 
  • #8
prehisto
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Ok I know the energy which is needed to feel something.
I assume that all hit the pupil and that diameter of an eye is d=15mm.
So i can calculate the solid angle.

But still how can I calculate power of LED ?
 
  • #9
voko
6,054
391
I think you can assume that the LED radiates in all directions. You have a particular solid angle where particular energy must be radiated to be be detectable. What else do you need?
 
  • #10
prehisto
115
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OK,thanks.
I think i got it.
 

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