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by banu


Optical fibres use a phenomenon known as total internal reflection to allow light to travel along a curved path. The light itself never actually bends, instead, it's reflected off the internal walls of the fibre. As long as the angle of reflection of the light is greater than what is called the critical angle it will travel quite far without much loss. Thus it's used as a way to send light (with coded information) over huge distances for communication and data services.

Video Overview

    • Scissors have functional sharp edges. Contact may result in injury.

    • Always keep blades away from fingers and body. Handle with care.

  1. Start by cutting the tube to make 2 rings of width about 1 cm each.
    • Start by cutting the tube to make 2 rings of width about 1 cm each.

    • Hold the AA battery and stretch the rubber band over it lengthwise.

    • Insert a safety pin head first in-between the rubber band and one of the battery terminals. Do the same for the other terminal.

    • Be mindful of which side of the battery has which terminal.

    • Using the previously mentioned steps make another battery holder for the other battery.

    • Tie a connecting wire from the negative pin of one battery to the positive pin of the other battery.

    • This will leave you with one free positive pin and one free negative pin.

    • Take a bit of tape, about 5 cm, and lay it flat lengthwise with the adhesive side facing you.

    • Then place the LED halfway on the tape, breadth-wise, as shown.

    • All of the clear plastic of the LED should be within the tape.

    • Now place the optical fibre next to the LED, such that one end of the fibre is touching the top of the LEDs plastic dome.

    • Roll up the tape. Make 3 turns to make sure that there is no light-bleed through the tape.

    • You now have your completed emitter, i.e. optic fibre connected to the LED. When powered on, the light will be channelled to the very tip of the fibre.

    • Start again with 5 cm of tape and place the LED along the tape, breadth-wise, such that only light from its top dome can escape.

    • Now roll it up, with the same number of turns you used for the other LED.

    • Cut the tape and you are left with an LED where only light coming form its top can be seen.

    • Now insert the LED into the bendable straw. Pick the opening that's closer to the bendable part.

    • Make sure that there is no light-bleed through the tape.

    • Cut the straw to about the length of the fibre optic wire, which is about 10 cm. This will make comparing the two simpler.

    • Hold both LED assemblies next to each other and observe their leads. Positive terminal is longer than negative terminal.

    • Proceed to twist both the shorter leads together, this will make a common negative. Also twist the longer leads together, that will be the common positive.

    • Connect a wire from the negative of the battery that we prepared earlier to the common negative of the LEDs.

    • Also connect a wire from the positive of the battery to the positive of the LEDs.

    • Both LEDs should now be illuminated.

    • Try bending the straw at an angle. Once you do this the light stops at the point at which the straw is no longer straight.

    • Bending the optic fibre, however, makes little difference to the light at its tip. In fact, even if you tie the fibre into a knot the light makes it to the end.

    • Observe that while the LED shining through the straw makes the whole straw glow, due to the light leaking out through the straw, on the other hand, the optic fibre path barely leaks any light along the fibre and most of the light is transmitted and is seen concentrated at its tip.

    • One or both of the LEDs is unlit. - This is possibly because of a loose wire or the LEDs have been connected in the reverse direction. Check all connections and terminals again.

    • The tip of the optic fibre isn't glowing. - This could be because the fibre has moved away from the LED's dome. You can push it back in or reattach the fibre again with tape.

    • If the LEDs don't light up, there is a high chance that the cells are not connected properly, or if you've played with it a lot, that the cells have discharged!

    • Total internal reflection is the only way to make light travel through the fibre (True/false)

    • Can you name anything else that only travels in a straight line?

    • What is the speed of light? a) 299,792,458 m/s b) 300,000 km/s c) 186,000 mi/s d) All of the above

    • There is nothing faster than the speed of light. (True/False)

    • Light only travels in straight lines. (True/False)

    • Series connection of optic fibre cable.

    • Parallel connection of multiple optic fibre cables.

    • Setting up a plastic pipe on the LED and comparing it with optic fibre.

    • To learn and make the variations for Explore Optic Fibre. Please click this link Explore Optic Fibre (Variations).

Finish Line

Kailash NR

Member since: 05/02/2017

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78 Guides authored