Specific heat of water is 4.2 J/g∘C . If light of frequency 3 x 109 Hz is used to heat 400 gm of water from 20∘C to 40∘C the number of photons needed will be
A
1.69×1029
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B
1.68×1028
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C
2.80×104
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D
2.80×105
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Solution
The correct option is B1.68×1028 C=4.2
f=3×109Hz
m=400gm
energy required
E=mc(ΔT)=400×4.2×20=33600J
each photon energy =hv=6.623×10−34×3×109=19.9×10−25J