Light can be thought of as either Waves or as particles. The wave approach fits the bill in considering Physical Optics, in which we like to discuss Wave Fronts (like the lines representing waves coming to shore).
In the study of Geometric Optics however, we take on light from the particle perspective, as Rays (the line, or perhaps arrow, that represents the direction that the wave it traveling). We often turn to the ray representation when considering geometric problems in optics, but both are handy.
When waves travel in homogeneous isotropic materials (that have the same properties in all regions and in all directions) the rays are all straight lines normal to the wave fronts.
One such material property is the Refractive Index “n” defined as the ratio of the speed of light in vacuum to that in the material This value is used extensively in Optics considerations.
All of us reading this have (ostensibly) been on this earth long enough to have experienced the weird ways that Light goes through things. Like, ever notice how the edges of your shadow aren’t actually well-defined? That’s Diffraction for ya.
Reflection and Refraction can get pretty gd complicated for objects as “simple” as a clear glass of water. We see these phenomena at most boundaries (the interface between materials) – along with other behavior, such as Polarization.
Course I say “most”, because there really ain’t too much refraction going on through solid objects, like concrete. That’s a phenomena for transparent objects.