When a hydrogen atoms going from \( n = 2 \) to \( n = 1 \) emits a photon, its recoil speed is \( \dfrac{x}{5} \, \text{m /s}^{} \). Where \( x = \) ________________. (Use, mass of hydrogen atom \( = 1.6 \times 10^{-27} \, \text{kg} \) )
