de Broglie Waves or Matter Waves and its Wavelength is discussed in this lecture. de Broglie waves are associated with a materialistic particle. A matter can show the dual nature: particle as well as the wave nature [ as you have seen in case of the light].  Dual nature of any materialistic particle is possible or you can observe it only when the de broglie wavelength is comparable to the size of the particle. de Broglie waves are observed only when a particle is in motion, here different cases are considered where one can observe the motion of light particles.