Concept: Optical fibers are long, thin glass strands with a diameter similar to that of a human hair. It is made up of a glass core and an outside coating that reflects light back into the core. The fiber is protected from external damage by a plastic covering on the outermost layer.
The total internal reflection principle is used by the optical fiber. Inside the optical fiber, light rays will have 100% internal reflection and will pass from one end to the other.
A portion of a light beam is reflected to the optically denser medium and the other part is refracted to the rarer medium when it travels through an optically denser medium to a rarer medium.
Internal reflection is the name given to this type of reflection.
After a particular angle of incidence of the light beam at the interface of the two media, refraction is no longer feasible, and the light beam is completely deflected. This is referred to as total inward reflection.
The critical angle is the angle of incidence at which the angle of refraction is 900. Total internal reflection is only possible if the angle of incidence is greater than the critical angle.
As a result, the angle of incidence must be greater than the critical angle for entire internal reflection to occur.
Hence, option 4 is correct.