The wave changes direction when passing from one medium to other due to change in the speed. This phenomenon is called as refraction. The change in direction of the wave can be quantified using the refractive indexes of the two materials. When a ray passes from denser medium (medium with higher refractive index) to rarer medium it bends or refracts away from the imaginary line, called normal, perpendicular to the surface. As the angle of incidence in denser medium becomes greater with respect to normal, the refracted ray bends further away from it.
At one particular angle, called critical angle, the refracted light does not enter rarer medium but instead travels along the surface between the two media. If the ray is incident at greater than critical angle, then the refracted ray reflects back into the denser medium resulting into total internal reflection. Thus, we can define total internal reflection as an optical phenomenon that happens when a ray of light strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface.
Two important conditions for total internal reflection are:
- Angle of incidence (i) should be greater than critical angle (ic).
- Ray should travel from denser medium to rarer medium.
Critical angle
The critical angle is the angle of incidence above which total internal reflection takes place. The angle of incidence is measured with respect to the normal at the refractive boundary. The critical angle can be found using Snell’s law, which states:
where, i is incident angle, r is refracted angle, n1 is refractive index of denser medium and n2 is the refractive index of rarer medium.
When angle of incidence is equal to critical angle the angle of refraction is 90°.
That is for i = ic, r = 90°
So the above equation changes to
(as sin(90°) = 1)
The critical angle, c, can therefore be found simply by knowing the refractive indexes of the two materials.
Total internal reflection can be observed while swimming, when one opens one’s eyes just under the water’s surface. If the water is calm, its surface appears mirror-like.
Optical fibers are based entirely on the principle of total internal reflection. An optical fiber is a flexible strand of glass used in communication. A fiber optic cable is usually made up of many of these strands, each carrying a signal made up of pulses of laser light. The light travels along the optical fiber, reflecting off the walls of the fiber. With a straight or smoothly bending fiber, the light will hit the wall at an angle higher than the critical angle and will all be reflected back into the fiber. Even though the light undergoes a large number of reflections when traveling along a fiber, no light is lost.
SchoolTutoring Academy is the premier educational services company for K-12 and college students. We offer tutoring programs for students in K-12, AP classes, and college. To learn more about how we help parents and students in Drummondsville visit: Tutoring in Drummondsville.