Register to reply 
Intensity, photons, human eye 
Share this thread: 
#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: 2,179

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 


Register to reply 
Related Discussions  
Relationship between light wavelength, intensity and photons energy  Introductory Physics Homework  3  
Effect of frequency & Intensity on number of photons  Advanced Physics Homework  3  
Sound Intensity and Human threshold for hearing  Introductory Physics Homework  1  
Shortest NonIonizing Wavelength for human body at any intensity/duration?  General Physics  4 