Q. A broken ligament is being ‘welded’ back in place using 20 ms pulses from a 0.5 W laser operating at a wavelength of 632 nm. The number of photons in 5 pulses of laser are
A) $1.59 \times 10^{- 18}$
B) $3.18 \times 10^{- 17}$
C) $1.59 \times 10^{23}$
D) $3.18 \times 10^{- 16}$
Explanation
(c) The power of laser is 0.5 W, let $n$ photon/s are incident by laser pulse on the broken ligament, then
$n\ \times hv = 0.5\ W$
$\Rightarrow \frac{nhc}{\lambda} = 0.5\ \Rightarrow n = \frac{0.5 \times \lambda}{hc}$
$= \frac{0.5 \times 632 \times 10^{- 9}}{6.626 \times 10^{- 34} \times 3 \times 10^{8}} = 1.59 \times 10^{18}$ photon/s
So, number of photons contained in 5 pulses are,
$n \times 5 \times \left( 20 \times 10^{3} \right) = 1.59 \times 10^{23}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.