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Find the number of photons emitted per second by a source of power 25W. Assume, wavelength of emitted light `= 6000Å. h = 6.62 xx 10^(-34)J.s`

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`lamda = 6000 Å = 6000 xx 10^(-10) m, c = 3 xx 10^(8) m.s^(-1)`
Number of photons per second emitted by a source of power P is,
`n = (P)/(hf) = (P)/(hc//lamda) = (P lamda)/(hc)`
`:. n= (25 xx 6000 xx 10^(-10))/(6.62 xx 10^(-34) xx 3 xx 10^(8))`
`= 7.55 xx 10^(19)`
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