Positron is the antiparticle of

What will be the velocity of a particle when its mass becomes twice its rest mass?

The momentum of a photon with energy $20 \, eV$ is

Find the number of photons emitted per second by a $10 \ kW$ radio transmitter operating at a wavelength of $500 \ m$.

What is the energy and momentum of a photon having frequency $v$?

In an accelerator experiment on high-energy collisions of electrons with positrons,a certain event is interpreted as annihilation of an electron-positron pair of total energy $10.2 \; BeV$ into two $\gamma$-rays of equal energy. What is the wavelength associated with each $\gamma$-ray? $(1 \; BeV = 10^9 \; eV)$