SCIENCE

law of refraction:

Bending Light

If you have ever half-submerged a straight stick into water, you have probably noticed that the stick appears bent at the point it enters the water (see Figure 1.) This optical effect is due to refraction. As light passes from one transparent medium to another, it changes speed, and bends. How much this happens depends on the refractive indexof the mediums and the angle between the light ray and the line perpendicular (normal) to the surface separating the two mediums (medium/medium interface) (See Figures 2a and 2b.) Each medium has a different refractive index (see list below.) The angle between the light ray and the normal as it leaves a medium is called theangle of incidence. The angle between the light ray and the normal as it enters a medium is called the angle of refraction.

Snell's Law

In 1621, a Dutch physicist named Willebrord Snell (1591-1626), derived the relationship between the different angles of light as it passes from one transperent medium to another. When light passes from one transparent medium to another, it bends according to Snell's law which states:Ni * Sin(Ai) = Nr * Sin(Ar),
where:
Ni is the refractive index of the medium the light is leaving,
Ai is the incident angle between the light ray and the normal to the meduim to medium interface,
Nr is the refractive index of the medium the light is entering,
Ar is the refractive angle between the light ray and the normal to the meduim to medium interface.

Critical Angle

Using the refraction simulator, notice how the light bends toward the normal when the light enters a medium of greater refractive index, and away from the normal when entering a medium of lesser refractive index. Then notice what happens when you move the flashlight to an angle close to 90 or -90 degrees in the medium with a higher refractive index. As you approach the critical angle the refracted light approaches 90 or -90 degrees and, at the critical angle, the angle of refractions becomes 90 or -90 and the light is no longer transmitted across the medium/medium interface. For angles greater in absolute value than the critical angle, all the light is reflected. This is calledtotal internal reflection.