If 5% of the energy supplied to a bulb is irradiated as visible light, how many quanta are emitted per second by a 100 W lamp? (Assume wavelength of visible light as $5.6×10^{-5}cm$)

$1.4×10^{19}$

Energy radiated as visible light = $\frac{5}{100}×100=5J/s$

Let n be the number of photons emitted per second. Then,

nhv = E = 5

$∴n=\frac{5λ}{hc}=\frac{5×5.6×10^{-7}}{(6.62×10^{-34})(3×10^8)}=1.4×10^{19}$