How Much Power Should an LED Emit to Be Visible from 5 Meters?

[prehisto]

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

 

[voko]

What is your difficulty?

 

[prehisto]

I even don't know where to start here.
Some kind of idea would be helpfull.

 

[voko]

Each photon carries energy. How much?

 

[prehisto]

I know that much
E=hv and v=c/lambda so E=hc/lambda

 

[voko]

So you should be able to solve the problem.

 

[Antiphon]

You also need to know how many photons miss or hit the pupil. Perhaps they all bit because of lenses. Or not.

 

[prehisto]

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 ?

 

[voko]

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?

 

[prehisto]

OK,thanks.
I think i got it.