Photon Intensity & Distance Calculation

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SUMMARY

The discussion focuses on calculating the distance from a 100 W light source emitting light at a wavelength of 600 nm, where the eye can detect a minimum of 20 photons per second. The energy per photon is determined using the equation E = hc/λ, leading to a calculated energy of approximately 3.31 x 10-19 J per photon. To find the distance at which the light can be detected, participants suggest considering the light's intensity over a spherical surface area as it radiates from the source.

PREREQUISITES
  • Understanding of the equation E = hf = hc/λ for photon energy calculations
  • Knowledge of light intensity and its relationship with distance
  • Familiarity with spherical geometry in the context of light propagation
  • Basic principles of photometry and detection thresholds for the human eye
NEXT STEPS
  • Calculate the energy per photon using E = hc/600 nm
  • Explore the concept of light intensity decay with distance in a spherical distribution
  • Learn about the inverse square law as it applies to light sources
  • Investigate photometric units and how they relate to human vision sensitivity
USEFUL FOR

Students in physics, optical engineers, and anyone interested in the principles of light detection and intensity calculations.

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Homework Statement


Supopse that a 100 W source radiates light of wavelength 600 nm in all directions and that the eye can detect this light if only 20 photons per second enter a dark adapted eye having a 7 mm diameter pupil. How far from the source can the light be detected under these conditions?


Homework Equations


E = hf = hc/\lambda

The Attempt at a Solution



I am having a hard time finding a relationship between the 100 W source and a distance. At what rate do the photons die off?

I can calculate the energy per photon: E = hc\ 600nm but with that information, I still don't see a way factor distance in there.

Anyone have some hints?

how I can figure out at what rate it will dissipate over a distance.
 
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jesuslovesu said:
I can calculate the energy per photon: E = hc\ 600nm but with that information, I still don't see a way factor distance in there.

Hi jesuslovesu! :smile:

Hint: imagine a sphere of radius r round the source.

How much energy falls each square metre of its surface? :smile:
 

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