The emission of light from a radioisotope occurs during what decay.

There are three types of decay which are named as alpha decay,beta decay and gamma decay.

Alpha decay: During alpha decay,the heavy unstable nucleus will emit  alpha particle which is a doubly charged helium nucleus.So here a particle is produced.

Beta decay: During beta decay ,the unstable nuclei will emit beta particles which are nothing else except the stream of electrons.

Gamma decay: As per the laws of radio activity, gamma decay is always followed by alpha and beta decay.During gamma decay,the daughter nuclei so formed after either alpha or beta decay will jump from higher excited to ground state with the release of energy called gamma radiation.

Hence the emission of light from a radioisotope occurs during gamma decay.

the part of the wave he is travelling on is the crest