Workshop Tutorials for Technological and Applied Physics
WR6T: Reflection and Refraction
A. Qualitative Questions:
1. Modern communication relies on optical fibres- for example high speed internet connections, phone lines and cable TV, all use fibre optic cables.
a. Explain how light is transmitted along optical fibres so that you can watch cable TV.
b. For an optical fibre cable to work, it must have a cladding. What can you say about the relative sizes of the refractive indices of the fibre core and the cladding? Which must be greater for the cable to work?
c. You have had a cable TV box installed and the cable guy has attached too long a cable. He rolls up the excess and pokes it under the TV, leaving just enough hanging out to reach the wall socket. Explain using diagrams why the cable will not work if it is too tightly wound up.
2. It is possible to see the sun or even a distant boat on the ocean when it is below the physical horizon. Explain, using a diagram, how this is possible.
B. Activity Questions:
1. Prism
Shine the light through the prism.
What do you see going into the prism?
What do you see coming out?
Which is refracted (bends) more – light of long or short wavelength?
Sometimes after rain or when there is a break in the clouds you may see a rainbow. On a sunny day if you stand with the sun behind you , you can make a rainbow by spraying a mist of water from a hose.
Draw a diagram showing how the rainbow is formed by the droplets of water.
2. Bent pencil
Why does the pencil appear to be bent?
Draw a diagram showing how the light is bending in this case.
3. Losing your marbles
Pour the liquid into the container with the marbles in it.
Why do they appear to disappear?
What can you conclude about the refractive index of the marbles and the liquid?
4. Total internal reflection
Shine the light into the cable.
Can you see the light through the sides of the cable?
Where is the light going, and why?
C. Quantitative Questions:
1. A novelty beer glass has a very thick bottom with a fake fly imbedded in it 1.2 cm below the surfac of the glass, as shown below. The refractive index of the glass is 1.6, and the refractive index of the beer is 1.35. Draw a diagram showing the path of a light ray from the fly to a person looking into the glass. When the fly is viewed from above, with 6.0 cm of beer in it, what is the apparent depth of the fly below the surface of the beer?
3. A fibre optic cable which is to carry a signal for an internet connection has a cladding with a refractive index of 1.52. The core glass fibre has a refractive index of 1.57.
At the connection to the computer, a beam of light enters the fibre from air at an angle q to the axis of the cable.
What is the maximum angle, q, that will allow the beam to propagate down the cable?
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The Workshop Tutorial Project –WR6T: Reflection and Refraction