Definition of Critical Angle
The critical angle is the specific angle of incidence in an optically denser medium for which the angle of refraction in a less dense medium is 90 degrees. When light rays strike the interface between these two media at this precise angle, the refracted ray travels along the boundary separating the two media.
Key Principles of Total Internal Reflection
This phenomenon occurs only when light attempts to pass from a denser medium (e.g., glass, water) to a less dense medium (e.g., air). For total internal reflection to take place, the angle of incidence must be greater than the critical angle, ensuring that no light is transmitted across the boundary, and all of it is reflected back into the denser medium.
A Practical Example
Consider light traveling through water towards the surface. If the light hits the surface at an angle greater than the critical angle for water-air (approximately 48.6 degrees), it will not exit into the air but will instead reflect back into the water, similar to looking at a mirror. This principle is what keeps light trapped inside fiber optic cables.
Importance and Applications
Understanding the critical angle is fundamental to numerous optical technologies. It is the core principle behind the functionality of fiber optic cables, which are essential for high-speed internet and medical endoscopes. It also plays a role in periscopes, binoculars, and gem cutting, influencing how light is guided and manipulated for various applications.