Electromagnetic waves have a wave and partical nature.This is known as wave duality.
1) Describe the situation in which the particals interact with the molecules
2)Describe a situation in which electro magnetic radiation is considered as a wave
For common everyday particles, such as marbles, salt and sand, their wavelengths are so small that their location can be accurately measured. For atoms and electrons, this becomes less clear.
In the diffraction experiment the electron wavelength is large so the location of the electron is very uncertain. The electron actually travels through both slits at once, just like a wave. In terms of particles it becomes impossible for us to really imagine this because it conflicts with everyday experience.
Define light as a collection of one or more photons propagating through space as electromagnetic waves. This definition, which combines light's wave and particle nature, makes it possible to rethink Thomas Young's double-slit experiment in this way: Light travels away from a source as an electromagnetic wave. When it encounters the slits, it passes through and divides into two wave fronts. These wave fronts overlap and approach the screen. At the moment of impact, however, the entire wave field disappears and a photon appears.
This shows that light some times behave as wave and some time as particle.