Two hydrogen atoms collide in a head on collision and end up with zero kinetic energy. Each then emits a photon of 121.6 nm (n=2 to n=1 transition). At what speed were the atoms moving before the collision?

Explanation:

Expression for the kinetic energy is as follows.

         K.E =

Now, total kinetic energy will be as follows.

    K.E = =

Since, this energy converts into electromagnetic radiation of wavelength 121.6 nm.

Relation between energy and photon is as follows.

   Energy of photon =

                                =

                                 =

          v =

             = m/s

Thus, we can conclude that atoms were moving at a speed of m/s before the collision.

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