Monochromatic light of wavelength $ 632.8\; nm$ is produced by a helium-neon laser. The power emitted is $9.42 \;mW$.
$(a)$ Find the energy and momentum of each photon in the light beam,
$(b)$ How many photons per second, on the average, arrive at a target irradiated by this beam? (Assume the beam to have uniform cross-section which is less than the target area), and
$(c)$ How fast does a hydrogen atom have to travel in order to have the same momentum as that of the photon?
Wavelength of monochromatic light $\lambda=632.8\, nm$
Power of He-be laser $P=9.42\, mW$
$(a)$ Energy of a photon is given by $E = hv$
Or $E=h c / \lambda$
Which gives $E=3.14 \times 10^{-19}\, J$
Now momentum of a photon $p=h / \lambda$
$p=1.05 \times 10^{-27}\, kg m / s$
$(b)$ For a beam of uniform cross-section having cross-sectional area less than target area
$P=E x N$
where $P =$ power emitted
$E =$ energy of photon
$N =$ number of photons Therefore
$N = P / E$
Substitution gives
$N =3 \times 10^{16}$ photons / second
$(c)$ Momentum of $He-Ne$ laser $=1.05 \times 10^{-27} \,kg m / s$
For this much momentum of a hydrogen atom $mv =1.05 \times 10^{-27}$
$v=1.05 \times 10^{-27} / 1.6 \times 10^{-27}$
$v=0.63\, m / s$
The required speed for hydrogen atom is $0.63\, m / s$
An electron with (rest mass $m_{0}$ ) moves with a speed of $0.8 c$. Its mass when it moves with this speed is
Assertion : The energy $(E)$ and momentum $(p)$ of a photon are related by $p = E/c$.
Reason : The photon behaves like a particle.
A Laser light of wavelength $660\,nm$ is used to weld Retina detachment. If a Laser pulse of width $60\, ms$ and power $0.5\, kW$ is used the approximate number of photons in the pulse are : [Take Planck's constant $h\, = 6.62\times10^{- 34}\, Js$]
Which of one is correct
Photo cell is a device to