Hello Shivani,

In the annihilation of an electron with a positron, the masses of both are converted to electromagnetic energy. Because we have assumed that before annihilation electron and positron are moving with negligible kinetic energy. We have to consider their rest masses.

2m(e)c^2 or 2m(po)c^2 = hf

Here, c = speed of light i.e 3*10^8 m/s, h = plank's constant i.e 6.6*10^-34 Js, f = frequency of resulting radiation in Hz.

f = (2*9.11*10^-31)(3*10^8)^2/(6.6*10^-34)

This value of f falls in gamma ray of EM Spectrum.