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Intensity, photons, human eye 
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#1
Apr409, 05:27 PM

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 nightadapted 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 


#2
Apr310, 02:10 AM

P: 2

your equation is correct
check ur calculations^_^ 


#3
Apr310, 03:37 AM

P: 2,195

I got the same numbers. Why do you think it's wrong? What do you think the right answer is?



#4
Apr310, 12:12 PM

P: 2

Intensity, photons, human eye
I got 1.150429803e13 for the intensity of the star
divide it by 1400, the final answer should be 8.217e17 


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