Define the terms: Critical angle, Acceptance angle and Attenuation.

Critical angle (\(\theta_c\)): The angle of incidence in the denser medium (the core) at the core–cladding interface for which the angle of refraction is 90°. For angles greater than \(\theta_c\), total internal reflection occurs. \(\sin\theta_c=\dfrac{n_2}{n_1}\).

Acceptance angle (\(\theta_a\)): The maximum angle to the fibre axis at which a ray can enter the fibre and still be guided along the core by total internal reflection. \(\sin\theta_a=\dfrac{\sqrt{n_1^2-n_2^2}}{n_0}\). The cone of half-angle \(\theta_a\) is the acceptance cone.

Attenuation (\(\alpha\)): The loss of optical power as light travels along the fibre, caused by absorption, scattering (Rayleigh) and bending losses. It is expressed in dB/km:

\[ \alpha=\frac{10}{L}\log_{10}\frac{P_{in}}{P_{out}}\ \text{dB/km} \]