How is the critical angle related to total internal reflection?

In optics, the critical angle serves as the threshold for total internal reflection. When a ray of light travels from a medium with a higher refractive index toward one with a lower index, it is typically both reflected and refracted at the boundary.12

However, the critical angle is defined as the maximum angle of incidence at which light can still be refracted into the second medium. If the angle of incidence exceeds this critical value, refraction becomes impossible, and the light undergoes total internal reflection, meaning the entire beam is reflected back into the original medium. Because refractive indices depend on the wavelength of light, the critical angle—and consequently the onset of total internal reflection—varies slightly depending on the color of the light.123