Rankers Physics

Photoelectric Effects and deBroglie Equation: Practice Problem & Solution

Monochromatic light of wavelength 667 nm is produced by a helium neon laser. The power emitted is 9 mW. The number of photons arriving per sec. on the average at a target irradiated by this beam is: (2009)
$3 \times 10^{16}$
$9 \times 10^{15}$
$3 \times 10^{19}$
$9 \times 10^{17}$

Solution Explained:

To solve this problem, we apply the core principles of Photoelectric Effects and deBroglie Equation. Understanding the underlying formula is key to arriving at the correct answer below:

Number of photons per second is $n = \frac{P}{E} = \frac{P \lambda}{h c}$. Substituting the values, $n = \frac{9 \times 10^{-3} \times 667 \times 10^{-9}}{6.6 \times 10^{-34} \times 3 \times 10^8} = \frac{6003 \times 10^{-12}}{19.8 \times 10^{-26}} \approx 3 \times 10^{16}$ photons per second.

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