A stationary hydrogen atom in the first excited state emits a photon. If the mass of the hydrogen atom is $m$ and its ionization energy is $E$, then the recoil velocity acquired by the atom is [speed of light = $c$]

• Correct answer
  A

  $\left[\sqrt{\frac{3E}{2m}+c^2}\right]-c$
• B

  $\left[\sqrt{\frac{3E}{4m}+c^2}\right]-c$
• C

  $\frac{3E}{4mc}$
• D

  $\frac{E}{mc}$

Hence, the correct answer is option: 'A'.