3.20 describe the role of total internal reflection in transmitting information along optical fibres and in prisms
Total internal reflection: When light falls on the surface of a lighter medium from denser medium at an angle of incidence greater than critical angle, then the light does not refracts. It rather reflects in the self-medium. This type of reflection is called total internal reflection.
Condition of total internal reflection:
- Light should fall in the surface of lighter medium from denser medium.
- Angle of incidence must be greater than the critical angle.
Uses of total internal reflection:
The prismatic periscope
Light passes normally through the surface AB of the first prism (that is, it enters the prism at 90oo. The critical angle for glass is 42o so the ray is totally internally reflected and is turned through 90o. On emerging from the first prism the light travels to a second prism which is positioned such that the ray is again totally internally reflected. The ray emerges parallel to the direction in which it was originally travelling.
The final image created by this type of periscope is likely to be sharper and brighter than that produced by a periscope that uses two mirrors. Because in mirrors, multiple images are formed due to several partial internal reflections at the non-silvered glass surface of the mirror.
Optical fibres
Optical fibre uses the property of total internal reflection. This is very thin strand composed of two different types of glass. The inner core is more optically dense than the outer one. As the fibres are narrow, light entering inner core always strike the boundary of the two glasses at an angle greater than critical angle. This technique is used to send information very fast at the speed of light.
Optical fibres are used in endoscopes and telecommunications.