Wave meets Particle
In the micro-world of quanta, waves can collide with particles. During the collision they transfer a part of their energy. In consequence, their wavelengths change accordingly to the loss of their energy ΔE:
ΔE = E - E' = h ν - h ν'.
The figure above shows an example of the Compton effect. An electromagnetic wave interacts with an electron, i.e., an electrical charged particle. We are sure that before and after the collision we are dealing with a wave, but at the moment of collision the particle property of the electromagnetic wave becomes obvious when the electron is being hit. The energy loss of the incoming wave ΔΕ leads to an increased wavelength λ' = c / ν' of the emitted wave after the collision.