Total Internal Reflection of Light When Refraction Becomes

Total Internal Reflection of Light – When Refraction Becomes Reflection!!! • When light travels from a more dense medium into a less dense medium (eg glass to air), refraction and reflection of light can occur at the boundary. • What happens depends on the angle of incidence and the critical angle, as the following three diagrams show. 1. Angle of Incidence I is (SMALL) less than the critical angle normal glass I R light ray Most of the light is refracted at the boundary. 2. Angle of Incidence is equal to the critical angle 3. Angle of Incidence is (BIG) greater than the critical angle Critical angle I=R I R The refracted ray travels along the boundary. [critical angle is about 420 for glass-air] I R All light is internally reflected at the boundary. This is Total Internal Reflection (TIR).

An optical fibre is a long, flexible rod, made of glass or transparent plastic, which is very narrow. Light sent in one end of an optical fibre travels through to the other end by undergoing total internal reflection (TIR) along its length. Refraction air glass TIR (angle of incidence is greater than critical angle) Refraction (angle of incidence is less than critical angle)

1. Telecommunications The light travelling down optical fibres can be encoded and used to carry information. Optical fibres have several advantages over similar thickness cables carrying electrical signals: • able to carry more information • less weakening of the signal as it travels along • more secure and reliable 2. Endoscopes These are devices used to view inside the human body. They consist of many fibres, half of which transmit light to the body part being viewed, and half transmit reflected light back to form and image. Spy cameras work in a similar way.

450 Reflecting Prisms to reflect through 900 or 1800 – better than mirrors since no light is absorbed and a brighter image is seen. 450 These are used in: Periscopes Binoculars Bicycle Reflectors