A monochromatic source of light operating at 198 W emits 4×1020 photons per second. Find the wavelength of light in nm.(Take h=6.6×10−34Js)
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Solution
Energy of single photon =hcλ If there are n photons, total energy =nhcλ power, P=Et=198W E=energy, t=time since t=1s, P=E (total ) Total energy emitted = 198 W Equating nhcλ=198 n=198λhc By substituting values: 4×1020=198×λ6.6×10−34×(3×108) λ=400nm