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A 2 mW laser operates at wavelength of 500 nm. The number of photons that will be emitted per second is : [Given Planck's constant h = 6.6 × 10 –34 Js, speed of light c = 3.0 × 10 8 m/s]

1. 5 × 10 15
2. 1.5 × 10 16
3. 1 × 10 16
4. 2 × 10 16

Correct answer: 5 × 10 15

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