Intensity, photons, human eye

In summary, the conversation discusses the estimation of the intensity of the dimmest star that can be detected by a night-adapted eye, based on the assumption that the human eye can pick up as few as 9 photons/s in the visible range. The intensity range of the eye is quite large, making it a very adaptable instrument. The formula used to calculate the intensity is N * E, where N is the number of photons per square meter per second, E is the energy of each photon, and the area is given by Pi * r^2. The final answer for the intensity of the star is 1.150429803e-13, and when divided by the given intensity of noon sunlight (1400 W/m
  • #1
Yroyathon
42
0
hi there. there's an example in the book, but I'm having a little trouble here.

Homework Statement


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.

Homework 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

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
 
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  • #2
your equation is correct
check ur calculations^_^
 
  • #3
I got the same numbers. Why do you think it's wrong? What do you think the right answer is?
 
  • #4
I got 1.150429803e-13 for the intensity of the star

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

1. What is intensity?

Intensity refers to the amount of energy per unit area, usually measured in watts per square meter (W/m²). It can also be described as the brightness or strength of a light source.

2. What are photons?

Photons are particles of light that carry electromagnetic energy. They are the smallest unit of light and can be described as both a particle and a wave.

3. How do photons affect the human eye?

Photons enter the eye through the cornea and are focused by the lens onto the retina. They then stimulate the photoreceptor cells, specifically the rods and cones, which then convert the light into electrical signals that are sent to the brain for processing.

4. Can the human eye detect all types of photons?

No, the human eye can only detect a small portion of the electromagnetic spectrum, known as visible light. This includes the colors of the rainbow: red, orange, yellow, green, blue, indigo, and violet.

5. How does intensity of light affect the human eye?

The intensity of light can affect the human eye in different ways. High intensity light can cause temporary blindness or discomfort, while low intensity light may not be bright enough for the eye to detect. The intensity of light can also affect the perception of color and contrast. Too much or too little intensity can make it difficult to see objects clearly.

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