Question

Activity on total internal reflection

Answer 1

Objectives:
Students will-
refresh their understanding of reflection and refraction;
learn how total internal reflections occurs;
observe total internal reflection;
discuss the important applications of total internal reflection.
Laser light in a Water Tank

Material List:
aquarium (fish tank, 5 gallon size is ideal)
water
laser pointer (He-Ne, red/orange in color)
powdered milk (coffee creamer)
white poster board, at least 12" on a side to stretch across the top of the fish tank

Procedure:
This is a very nice set up to demonstrate both reflection and refraction.
Look here for laser pointer safety guidelines.
1. Fill the aquarium (clear glass) almost full of water; leave about 5 cm unfilled.
2. Mix in less than a teaspoon of powdered coffee creamer and stir. You may have to do this in stages to obtain the optimum amount, but if you put in too much, you will have to start over.
3. Shine the laser pointer from the outside of one end of the aquarium from near the bottom pointing so the light will come out through the surface of the water (see photo below). You should be able to see the red path of the laser as it passes through the water. You will not be able to easily see laser light pass through clear water, but the powder adds larger masses that will scatter the laser light so you can see it. You should be able to see the light reflect off the top water surface. If you shine the laser pointer towards the side of the aquarium, you may be able to see light reflected off the glass side.
4. You will not be able to see light coming out of the aquarium into air, because air molecules are too small to effectively scatter visible light. However, you can place the white poster board over the top and sides of the aquarium and see the laser light shine on the white board.
5. If the laser light is quickly attenuated inside the aquarium, you have put too much coffee creamer in the water. Start over. If you can't see the light or if you can barely see it, then you need to put more creamer in and stir. It may take some practice to get this just right. Measure what you put in so you can reproduce the amount later. This photo was produced using a more powerful laser than a laser pointer, so the light could easily be seen in the 5-gallon fish tank for the photo. We used less than one teaspoon of coffee creamer. The laser can be seen in the lower right of the photo (red spot). The path of the laser light cannot be seen until after it enters the water. The light moves to the left and is totally internally reflected at the water surface. Note the straight lines that the laser light makes inside the aquarium. The fish tank can be seen as outlined by the cloudy area in the photo.
6. Shine the laser light so that it reflects off the water surface (from inside the water) at different angles. Use the poster board to see if light is being transmitted through the water/air interface (that is, is light being refracted?). You should easily be able to see the laser light reflect off the glass surface where it enters the aquarium (use poster board).
7. You should be able to find an angle (called the critical angle) where, as the angle with the water surface gets smaller, all the light will be reflected from the water surface back into the aquarium and none will be refracted out into the air. This angle should be about 40 degrees from the water surface for water and air. Smaller angles should have the light totally reflected back into the water. You will have to move the poster around on top of the aquarium to see the red spot, because the direction of the refracted moves dramatically with entrance angle.