What is light had always been mysterious for thinking people. Newton believed there are particles carrying light, explaining thereby why an obstacle in the light way creates a shadow on a screen. A century later, they discovered light is of wave nature. Now, think of a sum of waves of close frequencies. The waves interfere so the summing gives so called ‘wave packet’. A wave packet is a pattern linking waves and particles. It is localized in space, i. e. it moves changing its position and keeping finite length. Thus, a wave packet looks much like a particle, being a combination of waves though. Its speed can be different from that of waves contributing to it. Its amplitude is naturally greater the greater number of waves contribute to it. Speaking of its length, it either stays constant or increases with time, depending on the sort of waves. In quantum mechanics, what constitute a wave packet, are the waves of probability for a particle to be found in a specified region of space.