Particle Collision: Calculate Photon Wavelength

[Air]

Homework Statement

The results of particle collisions are observed using particle detectors. Sometimes the products of these collisions are themselves unstable and decay to give further particles; the products of decay can be photons.

In one collision, a stationary $\pi^\circ$ meson is produced which then decays to give two gamma ray photons. The rest mass of the meson is $135 \ MeV/c^2$.

$\pi ^\circ \rightarrow \beta + \beta$​

Calculate the wavelength of each photon.

Homework Equations

$E = hf = \frac{hc}{\lambda}$

The Attempt at a Solution

How can I find $E$? I can carry on from that point but I don't know which equation to use to find $E$?

 

[gendou2]

The energy of the stationary meson is calculated using E = mc²

E_pi = m_pi*c² = 135 MeV / c² * c² = 135 MeV