Optical Fibres – total internal reflection

Suppose you want to shine a torch beam down a long, straight hallway. Just point the beam straight down the hallway – light travels in straight lines, so there’s no problem. What if the hallway has a bend in it? You could place a mirror at the bend to reflect the light beam around the corner. What if the hallway is very winding with multiple bends? You might line the walls with mirrors and angle the beam so that it bounces from side-to-side all along the hallway. This is almost exactly what happens in an optical fibre, except that mirrors tend to scatter the light too much, so we arrange for it to be totally internally reflected inside layers of different glasses with reflective cladding around the outside.

Because the cladding does not absorb any light from the core, the light wave can travel great distances.

 In this image, only one layer of core is shown for greater clarity.

cladding-tir

Electrical signals and light can be transmitted for long distances using total internal reflection in a length of optical fibre, by using carefully chosen glasses with slightly different refractive indices, bundled together in lots of fibres. However, some of the light signal degrades within the fibre, mostly due to impurities in the glass.

If we want to see inside the body, an endoscope allows us to do so. A piece of optical fibre can direct light into body cavities and receive TV images back, enabling doctors to make accurate diagnoses.

endoscope

This is a well-lit endoscopy image from a fortunately healthy human stomach.

And finally…

Periscopes are devices to ‘see over’ from the Greek. Properly angled mirrors are OK to see over the fence at the football match, but submarines need a bit more clarity. periscope1

They use periscopes made from prisms, totally internally reflecting the light from the surface to the operator’s eye at the bottom of the picture. In exams, you need to be able to draw a ray diagram like this, getting the angles right

 

peri

The same trick was used in WW1 when soldiers needed to see over the trenches to keep an eye on the enemy.

 

There’s nothing really  new about periscopes. Johannes Gutenberg (he of the Gutenberg Bible) patented a design so that pilgrims could see over crowds at religious festivals. In 1430!


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