The Raman effect is a quantum effect observed when light strikes a material. The photons of laser light all have the same wavelength (the light is monochromatic), but they can undergo four different phenomena:
1. either they are transmitted, i.e. they pass through matter
2. or they are absorbed, i.e. their energy is totally transformed into heat by the atoms of matter
3. or they are reflected, i.e. they strike the material and are returned as by a mirror
4. or they strike the material and continue their path but after having taken or lost part of their energy to the atoms that they have struck. Because their energy has changed the wavelength of these photons has increased or decreased: they have undergone the Raman effect!
This effect is used in the telecommunications industry to amplify optical signals.