Question

A moving Helium $(He)$ atom absorbs a photon of wavelength $12.2\stackrel{{}^{\circ }}{A}$ and stops. What was the speed of Helium $(He)$ atom nearly?,
$\left(\text{Plank’s constant},h=6.626\times {\left(10\right)}^{-34}Js,\text{Mass of helium},m=6.646\times {\left(10\right)}^{-27}kg\right)$

• A. $20m{s}^{-1}$
• B. $80m{s}^{-1}$
• C. $120m{s}^{-1}$
• D. $40m{s}^{-1}$

Answer 1

Answer: (B)

The momentum of a photon, $p=\frac{h}{\lambda }$ .
where,
$h\to$ Planck's constant and
$\lambda \to$ wavelength.
Given, $\lambda =12.2\stackrel{{}^{\circ }}{A}$
By conservation of momentum,
$mv-\frac{h}{\lambda }=0$
$\Rightarrow v=\frac{h}{m\lambda }$
$\Rightarrow v=\frac{6.626\times 10^{-34}}{6.646\times 10^{-27}\times 12.2\times 10^{-10}}$
$\Rightarrow v=80m{s}^{-1}$