A typical incandescent light bulb emits approximately 3x108 visible-light photon

Question

A typical incandescent light bulb emits approximately 3x108 visible-light photons per second. Your eye, when it is fullydark adapted, can barely see the light from an incandescent lightbulb 10km away. How many photons per second are incident at the image point onyour retina? The diameter of a dark-adapted pupil is approximately7mm.
Give answer in terms of n. visible-light photons per second. Your eye, when it is fullydark adapted, can barely see the light from an incandescent lightbulb 10km away. How many photons per second are incident at the image point onyour retina? The diameter of a dark-adapted pupil is approximately7mm.
Give answer in terms of n.

Explanation / Answer

Power or number of photons as afunction of distance fromthe source is given as P(R) = N0/A where A=4R2 P(R) = N0/[4R2] The pupil area Ap = D2/4 so thenumbe rof photons enetering the pupil is np= ApP(R) =  [D2/4 ] N0/[4R2] np=   (D /R )2N0/16 np=   (0.07 /10,000 )23x108 /16 np=    9x10-4photons. P(R) = N0/[4R2] The pupil area Ap = D2/4 so thenumbe rof photons enetering the pupil is np= ApP(R) =  [D2/4 ] N0/[4R2] np=   (D /R )2N0/16 np=   (0.07 /10,000 )23x108 /16 np=    9x10-4photons.

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