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NEET 2012 (AIPMT, Prelims) · Photons: Energy, Momentum & Radiation Pressure

Photons: Energy, Momentum & Radiation Pressure question from NEET 2012 (AIPMT, Prelims)

Q32NEET 2012 · AIPMT · PRELIMSMedium

Photons: Energy, Momentum & Radiation Pressure

A 200 W sodium street lamp emits yellow light of wavelength 0.6 μ\mum. Assuming it to be 25% efficient in converting electrical energy to light, the number of photons of yellow light it emits per second is

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Answer: (a)

  1. Light output =25%= 25\% of 200 W=50 W200\text{ W} = 50\text{ W}
  2. Photon energy hcλ=6.6×10−34×3×1080.6×10−6=3.3×10−19 J\dfrac{hc}{\lambda} = \dfrac{6.6\times10^{-34}\times3\times10^{8}}{0.6\times10^{-6}} = 3.3\times10^{-19}\text{ J}
  3. n=503.3×10−19≈1.5×1020n = \dfrac{50}{3.3\times10^{-19}} \approx 1.5\times10^{20} per second
  4. So the answer is (a) 1.5×10201.5\times10^{20}.
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