# Power of LED and human eye

## 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

I even dont know where to start here.
Some kind of idea would be helpfull.

Each photon carries energy. How much?

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

So you should be able to solve the problem.

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

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 ?

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?

OK,thanks.
I think i got it.