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How many photons per second of radiation with wavelength lambda = 5 * 10⁻⁷ m must strike a perfectly absorbing blackened plate to produce a force of 6.62 * 10⁻⁵ N? (h = 6.62 * 10⁻³⁴ J s)

3 * 10¹⁹
5 * 10²²
3 * 10²²
None of these

Correct answer: 5 * 10²²

Solution

n = F*lambda/h = (6.62*10⁻⁵ * 5*10⁻⁷) / (6.62*10⁻³⁴) = 3.31*10⁻¹¹ / 6.62*10⁻³⁴ = 5*10²² photons/s.

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