Light rays move in straight lines. But through an optical fibre, they can move in any type of zigzag path because

Light typically travels in straight lines within a uniform transparent medium [1][3]. However, optical fibers utilize the principle of total internal reflection (TIR) to guide light along curved or zigzag paths [4]. For TIR to occur, light must travel from a denser medium (the fiber core) toward a rarer medium (the cladding) at an angle of incidence greater than the critical angle. This causes the light ray to reflect back into the core repeatedly rather than refracting out. These successive reflections allow the light to follow the fiber's geometry, even if it is bent or twisted, effectively moving in a zigzag manner through the core [4]. While scattering and absorption occur within the fiber, they are causes of signal loss (attenuation) rather than the mechanism for light propagation [4].

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > p. 134
2. [3] https://krct.ac.in/ktgadmin/assets/php/pdf/1576331029.pdf
3. [4] https://www.stannescet.ac.in/cms/staff/qbank/ECE/Notes/EC8751-OPTICAL%20COMMUNICATION-39240767-EC8751%20-OPTICAL%20COMMUNICATION%20NKTES.pdf