Ncert Exercises & Solutions

Question 11.4 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 average, arrive at a target irradiated by this beam? (Assume the beam to have a uniform cross-section which is less than the target area), and

(a)
Hint: $E=\frac{hc}{\lambda}$
Step 1: Find the energy of each photon.
The photons have the energy as:

$\begin{array}{l} E = \frac{h c}{λ} \\ = \frac{6.626 \times 10^{- 34} \times 3 \times 10^{8}}{632.8 \times 10^{- 9}} \\ = 3.141 \times 10^{- 19} J \end{array}$

Step 2: Find the momentum of each photon.
$Momentum,~p=\frac{h}{\lambda}$
$\Rightarrow~p=\frac{6.626\times10^{-34}}{632.8}=1.047\times10^{-27}~kg~ms^{-1}$

$\begin{array}{l} \end{array}$
(b)
Hint: P=nE
Step: Find the number of photons.
P = nE
Therefore, n= P/E

$\begin{array}{l} = \frac{9.42 \times 10^{- 3}}{3.141 \times 10^{- 19}} \\ = 3 \times 10^{16} photons/s \end{array}$

(c)
Hint: p=mv
Step: Find the speed of hydrogen atom.
P= 1.047 x 10^-27 kg m/s
Momentum is given as:
p=mv
Where,
v= speed of hydrogen atom

Therefore, v = p/m

$= \frac{1.047 \times 10^{- 27}}{1.66 \times 10^{- 27}} = 0.621 m / s.$

NEET Information

courses

Company

Botany Questions

Chemistry Questions

Physics Questions

Zoology Questions