Question

How does light travel in a fibre optic cable? Draw the path of a light ray having an angle of incidence of 75 degrees on an optical fibre with index of refraction n2=1.5 and where air is the external medium or n1=1.

Can light ever leave the cable? What would happen if the cable were to be bent?

What happens when a laser beam is passed through a prism? Is dispersion observed?

What does it mean to say light is monochromatic?

Considering light wave communication, why might it be useful to be able to separate different colours (frequencies) of light?

Answer 1

For n2=1.5 and n1 =1

sin(75)/sin(r)=1.5

sin(r) = sin(65)/1.5 = 0.644

r = 40.08 degrees

This ray when reaches the other end of tube, it gets refracted our with the same angle, 75 degrees.

For the light to leave the cable, it must be incident at an angle less than 41.8 degrees.

If the cable was bent, then it is possible that light will be incident at an angle less than 41.8 degrees and leave the cable.

Dispersion is observed when light with multiple wavelengths is incident on a medium.

Since laser beam is monochromatic(of single wavelength), dispersion is not observed.

White light on itself is not percievable by the human eye.

So, for light wave communication, we want to separate the colors from the white light.