In Davisson-Germer experiment, which particles are scattered by the $Ni-$crystal?
 

In Davisson and Germer's experiment, accelerating potential is kept constant at $54 \mathrm{V}$. As the detector is rotated, the first intensity maximum is obtained at an angle of

($e = 1.6\times {10}^{-19} \mathrm{C}$, $m$$=$$9.1\times {10}^{-31}$$\mathrm{kg}$)

Figure shows the energy levels $P, Q, R, S$ and $G$ of an atom where, $G$ is the ground state. A red line in the emission spectrum of the atom can be obtained by an energy level change from $Q$ to $S$. A blue line can be obtained by which of the following energy level change?