Hands-on Science Carnival 2010 Activity Stations: Light & Optics

Jell-O Optics, Total Internal Reflection and Fiber Optics

[Shopping List: clear unflavored gelatin; food colors; laser pointers; small LED flashlights; small white flashlight; strips of plastic to use as knives and spatulas; milk or non-dairy creamer powder; optics kit with flat plastic lenses and laser ray box; Lucite light pipe and fiber optic fountain; aquarium tank filled with water and a little non-dairy creamer or milk]

Jell-O Optics

1. Prepare large sheets of clear gelatin ~1/2" thick. Cookie pans work well; place sheets of wax paper in the bottom before filling with gelatin to make it easier to get the Jell-O out.
2. Cut the Jell-O into various shapes to make lenses and light pipes.
3. Use laser pointers, the ray box and small flashlights to observe how light beams travel into and out of- and through- Jell-O depending on the shape. Notice that the light beams bend at any air-Jell-O interface.
4. Make a long snake-like strip of Jell-O and point a laser or flashlight into one end. If the beam is pointed correctly, it should travel all the way to the other end, even if there are several fairly sharp turns. To make this easier, several shorter strips can be connected together rather than trying to make one very long strip. This is called a light pipe, and is very similar to what happens in an optical fiber (fiber optics).
5. Cut a triangle shape, then shine a small white flashlight beam towards one corner so that it bends and exits from the other side. You should see a rainbow spectrum of colors where it exits. It might help to let the light fall onto a small piece of white paper at the exit.
6. This can be done with the flat plastic lenses from the optics kit instead.

Total Internal Reflection and Frustrated Total Internal Reflection

1. Cut a long rectangular shape of Jell-O, or use a small square tank filled with water.
2. Shine a laser pointer beam into one side and out the opposite side. Starting with the beam perpendicular to the side, notice that as you increase the angle that you aim the beam initially (the incident angle), the angle of the beam inside the Jell-O (the angle of refraction) bends even more towards a line perpendicular to the surface.
3. Now watch the beam exiting the opposite side of the Jell-O. You should observe the opposite behavior as you change the incident angle of your beam now. Once the beam incident on the opposite side is greater than about 40 degrees from perpendicular, you should see a total reflection of the beam back into the Jell-O, as if you were shining it off a mirror, and no light will exit the opposite side at all. This is total internal reflection. It is what was happening above when you made the light bend to remain inside your light pipe even as it turned.
4. TIR in a fiber optic or light pipe can also be demonstrated with the coiled Lucite rod or fiber optic fountain. You can also demonstrate TIR with most of the flat plastic lenses, or an aquarium tank filled with water.
5. To observe Frustrated Total Internal Reflection, fill a square tank nearly full of water and looking down at an angle from above towards the back side of the tank. Place a small piece of paper with some writing on the back side of the tank and slide it down below the level of the water until you cannot see it any longer (you will only see a reflection on the bottom of the tank instead). You may need to move your eye position closer to the water surface to help this. Once you find a position where you cannot see the paper, pull it away, soak it in the water, and press it against the back side of the tank again in exactly the same position. Now it almost magically appears through the tank wall. You can also do this with your fingertip. At first it is not visible, but if you press it tightly against the tank wall you will see only the ridges on your fingerprint. If you wet your finger and press it against the back wall you will see all of you fingertip clearly.

What's Happening: When a beam of light passes from a less dense medium (such as air) into a more dense medium (such as water, glass, or on our case Jell-O), it is bent towards the normal (a line perpendicular to the surface). This is called refraction. As the angle of the beam is aimed towards the surface changes (the incident angle), the angle of the bent beam also changes (the refracted angle). Changing the angle or shape of the incident surface changes the incident angle at that point, which thus changes the angle of refraction. This is how lenses can be shaped to make light rays focus in desired ways. White light is actually a combination of all the colors in the visible light spectrum, and when a beam of white light is refracted, each of these colors actually bends a slightly different angle. This is called dispersion. A prism is just a piece of glass (or Jell-O) shaped to maximize the refraction and separate the component colors as much as possible.

Notice that as you aim your incident beam at the Jell-O surface closer and closer to being parallel to the Jell-O, the refracted beam inside the Jell-O will never make an angle any greater than about 40 degrees from perpendicular. This is called the critical angle, and is different for other materials. If you look at the beam inside the Jell-O as it hits the air surface on the back side of the Jell-O, you will find that for as you increase the incident and it approaches the critical angle the refracted beam outside the Jell-O becomes almost parallel to the Jell-O surface, and once the incident beam is greater than about 40 degrees there is no refracted beam at all, but instead a reflected beam inside the Jell-O. This is total internal reflection. Frustrated total internal reflection is a little complicated to explain completely, but it is a consequence of the fact that light rays that would otherwise be totally internally reflected can pass through the Jell-O-air interface if another denser material is very very close (within one or two wavelengths of light) to the interface. This property can be used in many interesting applications (besides the magically appearing paper trick), such as fingerprint sensors.

Variations: We may bring some glass lenses and prisms, as well as some plastic light pipes (an true optical fiber is actually a little different, instead of total internal reflection at a glass to air interface, it uses two different types of glass to create total internal reflection at the interface between the two glass types instead). Colored Jell-O could be used along with colored flashlights or laser pointers to demonstrate how light is absorbed and transmitted by colored materials (filters). For example, if we shine a green laser into green Jell-O, the beam passes through losing almost none of its initial intensity, but if we shine the green beam into red Jell-O, it is almost completely absorbed. Green materials are green because when white light shines on them they absorb all of the colors in the white light except green, which either passes through the material or is reflected back instead.