How can light be trapped? Teacher Notes

Transcription

1 How can light be trapped? Teacher Notes Concepts: (1) Light can be reflected. [ ] (2) The reflection of light can be engineered in such a way to send information long distances. The number of jobs created by this technology affects all of us. [ ] (3) LASER light is different from normal white light and must be treated with respect. [ ] Questions to get students thinking before the activity or to use after: Is there a way to trap light in a transparent material? Light travels in straight lines. How can it bend in a tube or cable? Part 1 - The reason these two activities are split is very simple. You do not want your students to use the wrong light! By separating them, this will not be a problem. Materials per group: 2 Liter clear soda bottle and top water - from the tap LASER pointer - class IIIA LASER pointer (output less than 5 milliwatts) also has an LED flashlight on it, these are available at Dollar Tree stores a few drops of milk SAFETY NOTE: Be sure to go over the LASER safety rules again and watch the students carefully! [ ] CAUTION LASER SAFETY NOTE: If you use a laser pointer, you must give safety instruction to everyone. Use only a Class II or IIIa laser pointer with less than 5 milliwatts output power - the lower the better! Laser pointer rules: Before turning on the laser pointer, always be sure that it is pointed away from yourself and others. Never look directly into a laser pointer. Never direct a laser pointer at another person. Do not look at a direct reflection of laser light from shiny surfaces. AVOID DIRECT EXPOSURE TO BEAM CLASS IIIa LASER PRODUCT Procedure: This is easy and fun to do. You will hear exclamations of surprise when the laser beam is all of a sudden reflected back into the water.

2 ! Observations/Results: This is what their diagrams should look like: Summing Up: 1.Why do you have to add a little bit of milk to the water? We can t see light until it interacts with something. The light interacts with (is reflected off) the milk particles and we see it. There isn t anything in the clean water to interact with light so we couldn t see it without the milk. 2.Water is transparent. You have seen laser light pass through water. Is it possible to trap light in water? Yes! Part 2 - The reason these two activities are split is very simple. You do not want your students to use the wrong light! By separating them, this will not be a problem. Materials per group: small LED flashlight - small finger flashlights are available at Dollar Tree (can be ordered online and shipped to the store) or online, many key chains have one attached fiber optic cable - a 6 to 12 length per group, can be ordered from fiber optic core - If at all possible, try to strip some of the cable. By putting just the core up to the LED light, students can observe the light escaping only from the end of the fiber. No light escapes from the sides of it. You can see this (not as easily) with a fiber from a small finger fiber optic flashlight. Procedure: Students will be able to easily follow the directions. Observations/Results: The light exits the cable and can be observed as a spot of light the same color as the flashlight. No light escapes from the sides of the fiber, even though it is transparent!

3 Summing Up; 1.Because the light energy travels down the cable with little energy loss, fiber is a great way to send information long distances. Use the illustration to help you describe total internal reflection. Total internal reflection means all the energy or light is kept in a material. 2.The diagram can also help you describe why little energy is lost. Try it. Use complete sentences in your description. If you look at the arrows, they are all the same color and there are no arrows that get out of the cable. That shows energy or light isn t lost. The light is all reflected like we saw with the soda bottle and water. 3.Describe why the light energy is able to pass through the cable even when the cable bends. It doesn t matter how the cable bends, the light or energy is still bounced or reflected down the cable.

4 ! How can light be trapped? Purpose: The purpose of this activity is to demonstrate how light can be trapped in a transparent material. Materials: 2 Liter clear soda bottle and top, water, LASER pointer, a few drops of milk AVOID DIRECT EXPOSURE TO BEAM Procedure: 1.Fill the soda bottle with water. Keep the water level below the curved part. 2.Add a few drops of milk and cap the bottle. Swirl a little to mix the milk throughout the bottle. 3.a)Aim the laser light straight through the bottle. Draw the path of the beam on the first diagram. b)change the angle of the laser by moving the back end down a bit and draw the path of the beam in the water. c)continue dropping the back end of the laser. Watch the path of the beam. All of a sudden something special will happen! The laser beam no longer goes through and out of the water. The beam reflects back into the water. Draw this on the third diagram. CLASS IIIa LASER PRODUCT Summing Up: 1.Why do you have to add a little bit of milk to the water? 2.Water is transparent. You have seen laser light pass through water. Is it possible to trap light in water?

5 How can light be trapped? - part 2 Purpose: The purpose of this activity is to demonstrate how light can be trapped in a transparent material. Materials: small LED flashlight, fiber optic cable, fiber optic core core plastic jacket Procedure: 1.In the last activity you investigated a phenomenon called total internal reflection. The light stayed in or was trapped by the water. All the light was reflected off the upper surface of the water. 2.Turn on the small LED flashlight. Hold one end of the fiber optic cable to the light. Check out the other end of the cable. What do you observe? 3.Gently bend the cable. Does the amount of light passing through the cable change? 4.If you have a piece of the transparent core, touch the core to the LED flashlight. Where do you see the light? Does the light pass through the sides of the transparent core? Summing Up; 1.Because the light energy travels down the cable with little energy loss, fiber is a great way to send information long distances. Use the illustration to help you describe total internal reflection. 2.The diagram can also help you describe why little energy is lost. Try it. Use complete sentences in your description. 3.Describe why the light energy is able to pass through the cable even when the cable bends.