Question

A moving Helium $\left(\right.He\left.\right)$ atom absorbs a photon of wavelength $12.2\overset{^\circ }{A}$ and stops. What was the speed of Helium $\left(\right.He\left.\right)$ atom nearly?,
$\left(\text{Plank's constant} , h = 6 . 626 \times \left(10\right)^{- 34} J s , \text{Mass of helium} , m = 6 . 646 \times \left(10\right)^{- 27} kg\right)$

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

Answer 1

Answer: (B)

The momentum of a photon, $p=\frac{h}{\lambda }$ .
where,
$h \rightarrow $ Planck's constant and
$\lambda \rightarrow $ wavelength.
Given, $\lambda =12.2\overset{^\circ }{A}$
By conservation of momentum,
$mv-\frac{h}{\lambda }=0$
$\Rightarrow v=\frac{h}{m \lambda }$
$\Rightarrow v=\frac{6 . 626 \times 10^{- 34}}{6 . 646 \times 10^{- 27} \times 12 . 2 \times 10^{- 10}}$
$\Rightarrow v=80ms^{- 1}$