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Optoelectronics, Optics, Lights and Lasers > Non-Coherent Light Sources > Incandescent Lamps > 

How many photons per second shine on the book from a 75W bulb?

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Author Topic: How many photons per second shine on the book from a 75W bulb?  (Read 12303 times)
Unknown
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« on: June 27, 2011, 02:05:06 AM »
Electric power is typically given in units of watts (1W = 1J/s)

About 95% of the power output of an incandescent bulb is converted to heat and 5% to light. If 10% of that light shines onto your textbook. How many photons per second shine on the book from a 75W bulb?

Assume the photons have a wavelength of 550 nm.
I do not understand how you got the 299E6 on the denominator..
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oldprof
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« Reply #1 on: June 27, 2011, 05:08:43 AM »
Of the 75 W, .5% = .005 hits your textbook.

Thus p = 75*.005 = 0.375 W are hitting your book.

The energy of each photon is e = hc/L = ?

Thus the number of photon/sec hitting your book is N = p/e = pL/hc = .375*550E-9/(6.63E-34*299E6) = 1.04042E+18 photons/sec  ANS.
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