At the interface between the two materials, e.g. air and water, light may be reflected at the interface or refracted (bent) into the new medium.
For Reflection the angle of incidence = angle of reflection.
For Refraction the light is bent when passing from one material to another, at an angle other than perpendicular.
A measure of how effective a material is in bending light is called the Index of Refraction (n), where:
Index of Refraction in Vacuum = 1 and for all other materials n > 1.0.
Most minerals have n values in the range 1.4 to 2.0.
A high Refractive Index indicates a low velocity for light travelling through that particular medium.
Snell's law can be used to calculate how much the light will bend on travelling into the new medium.
If the interface between the two materials represents the boundary between air (n ~ 1) and water (n = 1.33) and if angle of incidence = 45°, using Snell's Law the angle of refraction = 32°.
The equation holds whether light travels from air to water, or water to air.
In general, the light is refracted towards the normal to the boundary on entering the material with a higher refractive index and is refracted away from the normal on entering the material with lower refractive index.
In labs, you will be examining refraction and actually determine the refractive index of various materials.