# Figure out the ? in the figure below

• Does v means frequency here? – Netham Dec 12 '16 at 13:35
• That is the part of the puzzle ;) – alDiablo Dec 12 '16 at 13:43

- $2$ refers to the mass of Deuterium so it represents inertia
- $4$ refers the minimum number of Hydrogen atoms required for a fusion to take place also called critical mass
- $f$ refers to frequency. Energy of a photon is directly proportional to it's frequency (Planck-Einstein relation), it the cause of change in energy
- $h$ refers to the Planck constant. The reduced Planck constant is the quantum of angular momentum of a Photon
- $8v$ refers to $(8 \times \tt{frequency})$. Energy of one photon is $hf$ by Planck-Einstein relation. Energy of the two photons will be $2hf$. Value of $h$ is $4.13 \times 10^{-15} \tt{\ eV}$. Since frequency of an emitted photon can vary, total energy emitted in one fusion reaction is approximately $2\times4\times f \times 4.13 \times 10^{-15}\tt{\ eV}$, which can also be written as ~$8f$ or $8v$ in the scientific notation.