Question

Define critical angle. Explain working of optical fibre.

Hint:

In optical fibres, total internal reflection ensures the light stays within the core, enabling efficient transmission over long distances.

Answer 1

Step 1: Critical Angle.
The critical angle is defined as the angle of incidence in a denser medium for which the angle of refraction in the rarer medium is 90°. Beyond this angle, total internal reflection occurs. Mathematically, the critical angle \( \theta_c \) is given by: \[ \sin \theta_c = \frac{n_2}{n_1} \] where \( n_1 \) is the refractive index of the denser medium, and \( n_2 \) is the refractive index of the rarer medium.
Step 2: Working of Optical Fibre.
Optical fibres work on the principle of total internal reflection. The core of the optical fibre has a higher refractive index, and the cladding surrounding it has a lower refractive index. Light is injected into the core, and it undergoes total internal reflection, keeping the light confined within the core, even when the fibre is bent. This allows the transmission of light over long distances with minimal loss.
Step 3: Conclusion.
The critical angle defines the condition for total internal reflection, which is essential for the working of optical fibres.