The angle of incidence in the denser medium corresponding to which the angle of refraction in the rarer medium is 90º
The critical angle is the angle of incidence where the angle of refraction is 90 degree. The light must travel from an optically more dense medium to an optically less dense medium. The critical angle in optics refers to a specific angle of incidence. Beyond this angle, the total internal reflection of light will occur. The trajectory of a ray of light will deviate from the normal path when it strikes a medium that has a lower refractive index. As a result, the angle of exit of the ray is always greater than the angle of incidence. This type of reflection is called internal reflection.
The critical angle = the inverse function of the sine of (refraction index/incident index)
The equation is: θcric = Sin-1nr/ni