Critical Angle Expression
The critical angle is the angle of incidence at which the refracted ray of light travels along the boundary between two media. It is a significant concept in optics and is determined by the refractive indices of the two media involved.

Definition
The critical angle is denoted by the symbol θc and is calculated using the formula:
\[ \theta_c = \sin^{-1} \left( \frac{n_2}{n_1} \right) \]
where n1 is the refractive index of the medium in which the incident ray is traveling, and n2 is the refractive index of the medium in which the refracted ray is travelling.

Explanation
When light travels from a medium with a higher refractive index to a medium with a lower refractive index, the critical angle is the angle of incidence that causes the refracted ray to travel along the boundary between the two media. If the angle of incidence is greater than the critical angle, total internal reflection occurs, and the light is reflected back into the first medium.

Significance
Understanding the critical angle is crucial in various optical applications, such as fiber optics, where total internal reflection is utilized to transmit light signals over long distances without significant loss.
In conclusion, the critical angle expression provides a mathematical relationship between the refractive indices of two media and the angle of incidence at which total internal reflection occurs. It is a fundamental concept in optics and plays a vital role in various technological advancements.