A light bulb that is rated at 60W actually produces only about 3 Wof visible lig

Question

A light bulb that is rated at 60W actually produces only about 3 Wof visible light, most of the rest of the energy being infrared( orheat).
(a) about how many visible photons does such a light bulb produceseach second? use the average value lamda ~ 550 nm.
(b) if a person looks at such a bulb from about 10 ft away, abouthow many visible photons enter the eye per second? (pupil has adiameter of about 1 mm)
(c) by how many power of 10 does this exceed the minimum detectableintensity, which is about 100 photons entering the eye persecond?

Explanation / Answer

         = 83*1017Photons/s
(b)At 10ft intensity is I =(3W)/[(4)(pi){(10ft)(0.3048m/ft)}2] Then the power entering the eyeis (3W)/[(4)(pi){(10ft)(0.3048m/ft)}2]* [(pi)(0.001m)2]             = 8.07*10-6W Then number of photons entering per secondis (8.07*10-6W)/[(6.626*10-34Js)(3*108m/s)/(550*10-9m)]             =2.23*1013Photons/s
(c) (2.23*1013)/(100)= 2.23*1011

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