Question

A $\pi$ meson at rest decays into two gamma rays,i.e. ${\pi }^O\to \gamma +\gamma$. Then, which of the following can happen ?

• A. The two $\gamma$ -rays have unequal energies
• B. Both are $\gamma$ -rays move in the same direction
• C. The two $\gamma$ -rays move in the directions opposite to each other
• D. The $\gamma$ -rays periodically approaching and receding from each other

Answer 1

The correct option is C The two $\gamma$ -rays move in the directions opposite to each other

Since a $\pi$ meson is at rest initially, thus the initial momentum of the system is zero.

So, from law of conservation of linear momentum, the final momentum of the system must also be zero.

To do so, the two gamma rays must move in opposite direction to each other so that the total momentum of the system remains zero after the reaction.