1. A 100-watt lightbulb radiates energy at a rate of 100 J/s (The watt, a unit o
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Question
1. A 100-watt lightbulb radiates energy at a rate of 100 J/s (The watt, a unit of power, or energy over time, is defined as 1 J/s.) If all of the light emitted has a wavelength of 525 nm, how many photons are emitted per second?
Express your answer to three significant figures.
2. The energy required to dislodge electrons from sodium metal via the photoelectric effect is 275 kJ/mol. What wavelength in nm of light has sufficient energy per photon to dislodge an electron from the surface of sodium?
Express the wavelength in nanometers to three significant figures.
Explanation / Answer
(1) Energy emitted per second = 100 J/s
Energy per photon = hc/wavelength
= 6.626 x 10^(-34) x 2.998 x 10^8/525 x 10^(-9)
= 3.784 x 10^(-19) J
Number of photons emitted per second = energy emitted per second/energy per photon
= 100/3.784 x 10^(-19)
= 2.64 x 10^20 photons per second
(2) Energy per photon = energy per mole/Avogadros number
= 275 x 1000/6.022 x 10^23
= 4.5666 x 10^(-19) J
Wavelength = hc/energy per photon
= 6.626 x 10^(-34) x 2.998 x 10^8/4.5666 x 10^(-19)
= 4.35 x 10^(-7) m = 435 nm
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