What is the critical angle in optics?

The critical angle is specifically defined as the angle of incidence in a denser medium at which total internal reflection occurs. When light travels from a medium with a higher refractive index to one with a lower refractive index, there is a specific angle beyond which all the light is reflected back into the denser medium, rather than being refracted into the less dense medium. This phenomenon is critical for applications such as fiber optics and optical devices, where maintaining light within a medium is essential for functionality.

The critical angle can be mathematically expressed using Snell's law, which relates the angles of incidence and refraction to the indices of refraction of the two media. When the angle of incidence exceeds the critical angle, total internal reflection occurs, resulting in no light passing into the second medium.

The other options describe different optical principles. The first option outlines the general behavior of light at an interface but doesn’t pinpoint the unique condition of total internal reflection. The third option refers to the angle of reflection but does not accurately connect to the critical angle concept, as reflection occurs at all angles of incidence. The fourth option incorrectly associates the critical angle with light brightness, which is not a relevant factor in defining the critical angle.