In the gamma-decay the nucleus emits electromagnetic rays (called  -photons), therefore this process is similar to the light emission of atoms. However, there are differences between the visible light emission and gamma-ray emission! First, the gamma photons are emitted by the nucleus, whereas the visible light is emitted by the electron shell of the atom. Second, the energy of the gamma-photons are much higher than that of the visible photons. Therefore, the wave-length of the gamma-rays is much shorter than that of the visible light. Gamma rays are dangerous because their penetration range is large. Only a thick bulk of matter composed of heavy atoms (e.g. lead) can reduce their intensities | 
