- #1

clamatoman

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## Homework Statement

(a)Estimate the number of photons per second emitted by a 100-W lightbulb, assuming a photon wavelength of 550nm.(b) A person can just see this bulb for a distance of 800m, with the pupil dilated to 7.5mm. How many photons per second are entering the pupil?

## Homework Equations

E=hc/λ

h=6.63*10^-34 J s

c=3.0*10^8 m/s

1eV=1.602*10^-19 J

1 watt second = 100 joules

## The Attempt at a Solution

For part (a)

E=hc/λ=(6.63*10^-34 J s * 3.0*10^8 m/s)/(550nm) =3.616*10^-19 J

convert to eV: (3.616*10^-19 J * (1eV/1.602*10^-19J)=2.257 eV per photon

100 watt sec = 100 joules

(100 joules * (1eV/1.602*10^-19J))/2.257eV) =

**2.77*10^20 photons**

For part (b):[/B]

For part (b):

Not excatly sure how to proceed. I made a circle and determined the percent of the circumference was the 7.5mm pupil then multiplied that by the total number of photons, but the answer I got did not mach the book.

C=pi*d=pi*1600m=5026.548m

(7.5mm/5026.548m)*2.77*10^20 photons = 4.13*10^14 photons.

The answer in the book is 6.1*10^9 photons.

How can I get to this answer?