SNC2D Light and Geometric Optics
Teacher Demo: Bending Light—Part I
Topics
total internal reflection (TIR) /Timing
preparation: 20 mindemonstration: 15 min
Specific Expectations
SNC2D
A1.1 formulate scientific questions about observed relationships, ideas, problems, and/or issues, make predictions, and/or formulate hypotheses to focus inquiries or research
A1.8 analyse and interpret qualitative and/or quantitative data to determine whether the evidence supports or refutes the initial prediction or hypothesis, identifying possible sources of error, bias, or uncertainty
A1.10 draw conclusions based on inquiry results and research findings, and justify their conclusions
E3.7 identify the factors, in qualitative and quantitative terms, that affect the refraction of light as it passes from one medium to another
Introduction
In this demonstration, a beam of laser light is forced to follow the curved path of a stream of water.
Materials
safety goggles2 L empty colourless plastic pop bottle (flat sides work better) with cap
nail
masking tape / water
2–3 mL of milk (optional)
platform to raise bottle
low-power laser pointer
container (to collect water from the bottle)
Safety Considerations
· Only use a low-power visible laser (a Class 1 or Class 2 laser) because the power output and beam characteristics are such that they will not damage the eye.
· Do not look into a laser beam of any type and ensure that the beam cannot be reflected into the eye at full intensity.
Procedure
Wear appropriate PPE: safety goggles.
The preparation could be done before the class begins.
1. Use the nail to punch a small hole in the side of the pop bottle.
2. Cover the hole with tape.
3. Fill the bottle with water.
4. (Optional) Add 2–3 mL of milk to the bottle. Seal the bottle with the cap and mix until the milk is evenly distributed. Remove the cap.Place the bottle on a small platform to enhance visibility. Remove the cap from the bottle.
Organize the class into groups of 2 to 4 students.
5. Aim the beam of the laser pointer through the side of the bottle, pointing toward the hole on the opposite side.
6. Predict/Explain
Ask each group to predict what will happen to the light when you remove the tape from the hole. Encourage all groups to provide a rationale for their predictions.
Fig.1 Set-up for Steps 6 and 8
7. Observe
Remove the tape and allow the water to flow into a sink or container.
8. Explain
Ask the student groups to reconvene and revise their rationales, if necessary. Challenge them to suggest a model to help explain their observations.
Disposal
The plastic bottle may be reused for future demonstrations. Otherwise, it should be recycled.
What happens?
The laser beam will follow the stream of water, passing straight through the water in the bottle and then following the water flowing out of the bottle.
How does it work?
In Step 4 (optional), the milk is added to help make the light beam more visible. The particles in milk are large enough to scatter the laser light randomly in all directions. Some of this reflected light reaches our eyes, allowing us to see the light beam in the bottle. This is known as the Tyndall Effect.
The light beam does not actually bend as it exits the bottle. Rather, it follows the water stream by ricocheting off the boundary between the water stream and air, much like a bullet being fired at an angle into a steel pipe. This light phenomenon is known as total internal reflection (TIR). TIR occurs when light, travelling in an optically dense medium such as water or glass, encounters the boundary with a less dense medium such as air at an angle so great that refraction no longer occurs. In stead, the light is reflected back into the dense medium (Fig.2).
Fig.2 Light travelling through a fibre by total internal reflection
Teaching Suggestions/Hints
1. The platform should be large enough to support the bottle and high that the class can view the demo clearly. A small sheet of plywood resting on wooden blocks or books would do.
2. The laser pointer must be aimed directly at the hole because its light does not diverge like the light emitted by a flashlight.
3. If you have difficulty holding the laser steady, attach it to a retort pole with an adjustable clamp. Plastic bottles with flat sides work best since aiming error is magnified by refraction in a cylindrical bottle. A slight aiming error changes the angle of incidence, thus causing different amounts of refraction. Notice, in the YouTube video (see Additional Resources), how carefully the person is holding the laser pointer.
4. In Step 7 some students may predict that the light will continue straight through the hole because they know that light travels in straight lines. A few may suggest that the light follows the path of the water because it reflects off the insides of the water stream. Accept these predictions and encourage critical thinking when developing a rationale.
5. In Step 9 a sample model might be a ball bouncing off the sides of a long rectangular box as it makes its way from one end of the box to the other.
6. Light travels along optical fibres due to total internal reflection. Demonstrate passing light through a bundle of optical fibres. These are readily available from science equipment suppliers. Otherwise, they can possibly be obtained from parents who work in the communications sector.
Next Steps
This demonstration launches a discussion about the applications of total internal reflection. Introduce phenomena such as why diamonds sparkle, how optical fibres are useful in telecommunications or medicine, how triangular prisms work in periscopes or binoculars, or how retro-reflectors work.
Additional Resources
1. Online video showing this demonstration -
http://www.youtube.com/watch?v=hBQ8fh_Fp04&feature=related
2. STAO Power Point: Total Internal Reflection (TIR) - http://stao.ca/VLresources/2008/TIR.ppt