# intensity, photons, human eye

by Yroyathon
Tags: human, intensity, photons
 P: 42 hi there. there's an example in the book, but i'm having a little trouble here. 1. The problem statement, all variables and given/known data Assume that the human eye can pick up as few as 9 photons/s in the visible range. Based on this, estimate the intensity of the dimmest star that can be detected by a night-adapted eye. What is the ratio of this intensity to the intensity of noon sunlight, some 1400 W/m2? This large intensity range means that the eye is indeed a very adaptable instrument. Answer format = (intensity of 9 photons/s / intensity of noon sun) use 3mm for the radius of the pupil. use 550 nm for wavelength. 2. Relevant equations I=Power/Area I=N*E, N is the number of photons/(m^2*s) E=h*f f=c/lambda A=Pi*r^2 3. The attempt at a solution So I want the intensity, which is N * E, which is (N * h * c) / (Pi * r^2 * lambda). I get the intensity of the dimmest star on the human eye as 1.14592 * 10^(-13). With the given intensity of noon sunlight (do I need to adjust this for the area of the pupil?...), I divide it. 1.14592*10^(-13) / 1440 = 7.95775*10^(-17). but this is wrong, so... either I was supposed to adjust the noon intensity, or I've made one or several other mistakes. I'd appreciate any insights or tips. Thanks! ,Yroyathon
 P: 2 your equation is correct check ur calculations^_^
 P: 2,035 I got the same numbers. Why do you think it's wrong? What do you think the right answer is?
P: 2

## intensity, photons, human eye

I got 1.150429803e-13 for the intensity of the star

divide it by 1400, the final answer should be 8.217e-17

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