A Quick Recap on Refraction of Light
- When light travels to a different medium, refraction will occur, and the light will change its direction of travel.
When light travels from a denser medium (higher refractive index) to a less dense medium (lower refractive index), light refracts away from the normal line.
When light travels from a less dense medium (lower refractive index) to a denser medium (higher refractive index), light refracts towards the normal line.
Total Internal Reflection
Let's now consider a case where light is travelling from a denser medium to a less dense medium (eg. water → air)
- The light will refract away from the normal line in the less dense medium.
- If you keep increasing the angle of incidence, the angle of refraction will become 90° at one point. The angle of incidence at this point is called the critical angle.
- At the critical angle, the refracted ray will travel along the surface of the boundary of two medium.
- At an incident angle larger than the critical angle, all of the light ray will be reflected back. This phenomenon is called total internal reflection.
Relationship between refractive index and critical angle (when light travels from a denser medium to air):
sin(c) = 1 / n
c = critical angle
n = refractive index of dense medium
Example 1
What is the critical angle for total internal reflection to occur when light travels from water to air? The refractive index of water is 1.33.
Solution.
sin(c) = 1 / 1.33 = 0.751
c = 48.8°
Optical Fiber
Total internal reflection can be a useful tool to confine light. One of the applications of total internal reflection is in optical fibers for transmitting light.
Optical fibers are usually made of outer cladding with low refractive index and inner core with high refractive index. This design will make it possible for total internal reflection to occur. In this way, light can travel long distances with minimal information loss. Optical fibers are often used in telecommunications.