Light travels from one end to the other of an optical fibre by total internal reflection, even when the fibre is bent. Very little light is absorbed in the glass. As none of the light refracts into the air, the ray has been totally internally reflected inside the material. If the angle of incidence is increased any more, then the ray does not exit glass into the air, but is reflected inside the glass. At any angle greater than the critical angle, total internal reflection occurs and the light ray obeys the normal rules for reflection (ie angle of incidence = angle of reflection). The minimum angle at which total internal reflection occurs is known as the critical angle (for Perspex this is about 43˚). When a ray of light leaves a denser medium, it moves away from the normal.Īs can be seen from the above diagram, as the angle of incidence inside the material increases, the angle of refraction increases, to a point at which it leaves the material at 90˚. Total internal reflection and the critical angle The optical fibre makes use of the 'total internal reflection' of light. Light can be transmitted through it over very large distances. An optical fibre is a thin strand of high-quality glass.
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