5.2.3 Fibre Optic Rod 1 PARTNERS: Purpose: To replicate how light behaves in fibre optic cables by using a laser light and a solid acrylic rod.

Transcription

1 5.2.3 Fibre Optic Rod 1 NAME: DATE: PARTNERS: Purpose: Materials: To replicate how light behaves in fibre optic cables by using a laser light and a solid acrylic rod. 1. Acrylic rod 2. Laser pointer 3. Protractor & ruler 4. Blank sheet of paper 5. Vaseline (optional) Diagrams: 1. Case 1 Incident light i 2. Case 2 Incident light i 3. Case 3 Incident light i

2 Procedure: Fibre Optic Rod 2 Fibre optic cables contain bundles of individual glass fibres that are approximately the same diameter as a human hair! Due to this small size it is very difficult to see how light is transmitted in these fibres, but we can replicate this effect by using an acrylic rod to represent a single fibre optic strand. Activity 1 How light behaves in a fibre optic rod Look at the 3 different cases in the Diagrams section. These diagrams illustrate three different ways that light can behave as it passes into and through an individual fibre optic strand. 1. One of these diagrams illustrates light loss. In this situation, light enters the glass fibre at an incident angle such that the light ray refracts back out of the glass. Hence, the light is lost. Which case shows light loss? 2. Another diagram illustrates the acceptance angle. Which case shows the acceptance angle? What do you think the term acceptance angle means? Brainstorm with your group to come up with a definition. 3. A third diagram shows a general example of total internal reflection, which is an ideal situation for how light should travel down a fibre optic cable. Which case represents total internal reflection?

3 Activity 2 The acrylic rod Fibre Optic Rod 3 1. With a pencil and ruler, draw a long horizontal line lengthwise on a piece of paper. 2. Lay the acrylic rod on the piece of paper, so its axis (lengthwise) lies directly on top of the horizontal line you drew on the paper. Refer to the Diagram section the dotted line represents this axis. 3. Your instructor will dim the lights. Lay the laser pointer flat on the table surface, and shine the beam of light so it hits the center of the rod, perpendicular to the fiber axis as shown below: laser a. What happens to the beam of light as it hits the acrylic rod? Sketch your observations below. b. If you shine the light perpendicular to the fibre axis, is this an ideal way to transmit a signal down a fibre optic cable? Explain.

4 5.2.3 Fibre Optic Rod 4 4. Shine the laser into one of the ends of the acrylic rod, as shown in the diagram below: Angle of incidence a. What seems to happen with the beam of light as it enters the acrylic rod? Where does the beam of light emerge back out of the rod? b. What is happening here? Is this a better way to transmit a signal down a fibre optic cable? How do you know that this scenario is better than what you saw in Question 3?

5 5.2.3 Fibre Optic Rod 5 c. (optional) Keep the laser pointed at the rod so you get total internal reflection. Put a dab of Vaseline on your finger, and dab the Vaseline on the rod at one of the points where the light reflects off the internal surface of the acrylic. Describe what happens, and brainstorm with your group as to why this is happening. Questions: 1. Fibre optic cables have to be made in a very clean environment. What would happen if the manufacturer accidentally introduced dust or dirt into a strand of fibre optic cable? What would happen if the fibre cracked? 2. Where are fibre optics commonly used? List some common applications.

6 5.2.3 Fibre Optic Rod 6 3. On the Enterprise, Scotty invents a fibre optic cable made of transparent aluminum instead of acrylic. If we replicated our experiment using Scotty s new material, what changes would you observe? 4. Consider a fibre optic cable that is made of a single type of material. It is shaped in an S shape. How does light travel from one end to another? Does the light actually bend in an S shape? Sketch how the light travels below. Is this consistent with the behaviour of light you ve observed so far in previous refraction & reflection experiments? References: Kezerashvili, R. Ya., L. Leng, Catching Attention in Fiber Optics Class, Accessed July 25, 2007.