Question

A certain helium-neon laser pointer, emitting light with a wavelength of 632 nm, has a beam with an intensity of 825 W/m2 and a diameter of 2.80 mm. How many photons are emitted by the laser pointer every second?

Answer 1

Answer:

$N = 1.62 \times 10^{15}$

Explanation:

Energy of each photon is given as

$E = \frac{hc}{\lambda}$

here we will have

$\lambda = 632 nm$

now we will have

$E = \frac{(6.626\times 10^{-34})(3 \times 10^8)}{632 \times 10^{-9}}$

$E = 3.14 \times 10^{-19} J$

now let say there is N number of photons per second

so power due to photons is

$P = N(3.14 \times 10^{-19})$

now intensity is given as power received per unit area

so we have

$I = \frac{P}{A}$

$825 = \frac{N(3.14 \times 10^{-19})}{\pi (1.40 \times 10^{-3})^2}$

$825 = N(5.11 \times 10^{-14})$

$N = 1.62 \times 10^{15}$