# Power of LED and human eye

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

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.