Diffraction
Occurs by edges, slits, apertures, etc.
Due addition, interference of different points on the wavefront - light travels by paths with different lengths.
Reflection, refraction can be visualized and mathematically described by Huagens' principles with continues the idea for diffraction.
Yong's double slit experiment shows wave properties of light, also with diffraction effects.
Diffraction grating - structure which diffract light into several beams to different directions.
Circular aperture - "Airy" disk with different angles by different sizes of the hole.