WAVE-PARTICLE DUALITY & the Experimental Observations to back it up:
Wave-particle duality states that all matter and energy exhibit wave-like and particle-like behaviour. This can be seen in several experiments.
Firstly Young's slit experiment, which involves sending collimated light through a double slit, with widths of the order of magnitude of the wavelength of the light, and measuring the interference pattern on a screen a short distance away. The intensity on the screen varies as cos2, meaning that at some angle the light from both slits is in phase and constructively interfering to form maxima, and that at other angles it is completely out of phase and destructively interferes to give intensity minima. This has been carried out with light of very low intensities so that you can see one photon at a time hit the screen, but the same interference pattern builds up over time. This suggests that each photon is acting like a wave. The experiment has also been done with neutrons and heavier particles and the same pattern results; again suggesting that these particles are acting as waves.
Secondly electrons fired at a crystal are seen ...
Wave-particle duality states that all matter and energy exhibit wave-like and particle-like behaviour. This can be seen in several experiments including Young's slits, the Davisson-Germer experiment (diffraction) and quantum mechanical tunnelling.