Concept:The bulb converts electrical power into light; the number of photons per second is the total light energy per second divided by the energy of one photon.
Chemical Equation / Formula:E=n×εwhere
E = light energy per second (J),
n = number of photons per second,
ε = energy per photon (J).
Explanation:Power of bulb = 150 W = 150 J/s.
In 1 second, electrical energy consumed = 150 J.
Only 8% of this energy becomes light.
Light energy per second =
150×1008=12 J.
Energy of one photon =
4.42×10−19 J.
Number of photons emitted per second:
n=4.42×10−1912=2.715×1019.
Rounded to three significant figures:
2.71×1019 photons per second.
Shortcut:Use the direct formula:
n=EphotonP×t×(% as light).
Substitute
P=150 W,
t=1 s,
8%=0.08,
Ephoton=4.42×10−19 J.
n=4.42×10−19150×1×0.08=2.71×1019.
Answer:A.
2.71×1019
