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Q: 11.4 (a) Monochromatic light of wavelength 632.8\hspace{1mm} nm is produced by a helium-neon laser. The power emitted is 9.42\hspace{1mm} mW Find the energy and momentum of each photon in the light beam.

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The energy of photons is given by the  relation

\\E=h\nu \\ E=\frac{hc}{\lambda }\\ E=\frac{6.62\times 10^{-34}\times 3\times 10^{8}}{632\times 10^{-9}}\\ E=3.14\times 10^{-19}\ J

Momentum is given by De Broglie's Equation

\\p=\frac{h}{\lambda }\\ p=\frac{6.62\times 10^{-34}}{632.8\times 10^{-9}}\\ p=1.046\times 10^{-27}\ kg\ m\ s^{-1}

The energy of the photons in the light beam is 3.14\times10-19 J and the momentum of the photons is 1.046\times10-27 kg m s-1.

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