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A parallel beam of light of wavelength 900 nm and intensity 100 Wm –2 is incident on a surface perpendicular to the beam. The number of photons crossing 1 cm –2 area perpendicular to the beam in one second is

1. 3 × 10 16
2. 4.5 × 10 16
3. 4.5 × 10 17
4. 4.5 × 10 20

Correct answer: 4.5 × 10 16

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