Light follows the "Path of Least Time" to get to a destination in the quickest way possible. This involves 2 different mediums and a "bend." This bend is otherwise known as refraction. Refraction is when light passes through a medium and changes its path. It follows the principle of Snell's Law, where n1(sin1) = n2(sin2). In Snell's Law, n1 is the index of the angle of incidence, angle 1 is the angle of incidence, n2 is the index of the angle of refraction and angle 2 is the angle of refraction itself.
Real World Connection:
The image above depicts refraction through a pencil in a container of oil. The upper half of the pencil remains in air, where the index of refraction is 1. The lower half of the pencil is submerged in oil, where the index of refraction is 1.47. Because the pencil is moving at a faster velocity in the air, it will slow down in the oil, making the angle if refraction smaller than the angle of incidence. If we plugged these numbers into Snell's Law, we would find the angle of refraction, which would explain the "bend" in the image shown above. A "bend" is necessary in order for light to achieve the path of least time.
