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JEE Main 2017 (9 Apr, Shift 1) · Photons: Energy, Momentum & Radiation Pressure

Photons: Energy, Momentum & Radiation Pressure question from JEE Main 2017 (9 Apr, Shift 1)

Q31JEE Main 2017 · 9 APR · SHIFT 1Easy

Photons: Energy, Momentum & Radiation Pressure

A Laser light of wavelength 660 nm is used to weld Retina detachment. If a Laser pulse of width 60 ms and power 0.5 kW is used the approximate number of photons in the pulse are: [Take Planck's constant h == 6.62 ×\times 10−-34 Js]

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Answer: (a)

  1. Pulse energy E=Pt=500×60×10−3=30 JE = Pt = 500\times 60\times10^{-3} = 30\text{ J}
  2. Photon energy hcλ=6.62×10−34×3×108660×10−9≈3×10−19 J\dfrac{hc}{\lambda} = \dfrac{6.62\times10^{-34}\times3\times10^{8}}{660\times10^{-9}} \approx 3\times10^{-19}\text{ J}
  3. N=303×10−19=1020N = \dfrac{30}{3\times10^{-19}} = 10^{20}
  4. So the answer is (a) 102010^{20}.
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