1. Lesson 9

2.  The phenomenon of refraction had been observed for centuries, but it was not until 1621 that its cause was stated mathematically.

3.  Willebrord Snell (1591–1626) was a Dutch astronomer and mathematician who is credited with identifying the exact relationship between the angle of incidence and the angle of refraction.

4.  Snell’s law is a formula that uses values for the index of refraction to calculate the new angle that a ray will take as a beam of light strikes the interface between two media.

5.  If you call the indices of refraction of the two media n 1 and n 2 and call the angles of incidence and the angle of refraction θ 1 and θ 2, then the formula for Snell’s law is:  n 1 sinθ 1 = n 2 sinθ 2

6.  Sometimes, such as in the case of fibre optics, light does not pass from one medium to another but stays within the medium.

7.  In total internal reflection, light reflects completely off the inside wall of a denser medium (higher index of refraction) rather than passing through the wall into a less dense medium (lower index of refraction).

8.  Recall that when light passes from a denser material, such as water, into a less dense medium, such as air, the light refracts away from the normal.

9.  As the angle of incidence increases, the angle of refraction increases. At a certain angle, called the critical angle, the refracted ray of light follows a path exactly along the surface of the water. Even though the light refracts, it does not leave the water. In a way, the light is “trapped” inside the water

10.  When light crosses a boundary between materials with different indices, the light beam will be partially refracted at the boundary surface, and partially reflected.

13.  Both total internal reflection and refraction play a role in forming a mirage.  A mirage is an image of a distant object produced as light refracts through air of different densities.

14.  Since the light rays pass through layers of air with progressively lower indices of refraction, eventually the light is totally internally reflected.