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

(a) 1.5×1020                                (b) 6×1018

(c) 62×1020                                 (d) 3×1019

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