Rotate the laser to increase the angle and note the angle at which the emergent ray travels along the flat face of the prism as shown in diagram (b).
Rotate the laser further and note what happens to the ray of light as shown in diagram (c).
Observation Conclusion
The critical angle can be measured from the protractor printed on the page if the prism is placed directly over it. Note
This experiment can be performed as Mandatory Experiment 17.The refractive index can be calculated using the formula ,where n is the refractive index and C is the critical angle.
n 1 sin C
Transmission of Light through Optical Fibres
Optical fibres are made from a glass core with a layer of glass cladding, usually with an outer plastic coating to protect it. Light is transmitted along the core.
> C
The signal is introduced at one end of the fibre. As the light travels along, it strikes the boundary between the denser core and less dense cladding at an angle greater than the critical for the two media. Total internal reflection occurs and the light continues travelling inside the core. This continues until the light reaches the end of the fibre and is able to escape.
Light signal 1 Light signal 2 Fig 13.10: Transmission of light through an optical fibre
The cladding prevents light escaping into adjacent fibres and also prevents damage to the surface of the core.A plastic sheath surrounds the cladding to provide further protection from damage.
SAMPLE PROBLEM 13D The refractive index of the glass in the core of an optical fibre is 1.5. Calculate the critical angle for glass.
