What is the critical angle?

The critical angle is specifically defined as the angle of incidence in a denser medium at which the refracted light ray travels along the boundary between the two media, resulting in an angle of refraction of 90°. This condition occurs when the refractive index of the denser medium exceeds that of the less dense medium.

When light travels from a medium with a higher refractive index to one with a lower refractive index, an increase in the angle of incidence eventually leads to a situation where the refracted angle reaches 90°; this is the critical angle. Beyond this angle, total internal reflection occurs, meaning that all the light is reflected back into the denser medium rather than refracting into the less dense medium.

Understanding the critical angle is pivotal in applications like fiber optics, where total internal reflection is utilized to transmit light over long distances without significant loss.