PHO 101 Photonics Concepts
Three Rivers Community College ONLINE
Home Lab #3
Materials: Jello® Jigglers, tooth picks or non-serrated knife for cutting (a cleaver makes a nice, straight cut), laser pointer, protractor and ruler
Procedure: Read through all steps before proceding.
1. First you need to make the Jello. The object is to have a rectangle of Jello about at least 2 cm thick and 4-5 cm square. The length is not too important, but the edges that light will travel through need to be very straight and flat. You may need to cut a few pieces to get it right. (Figure 1a)
Figure 1a. Jello Block with Smooth Edge
Figure 1b. Photo of Jello block and pan. (Pan is approximately 5 cm deep.)
The recipe for Jigglers is on the Jello® brand package. You can use half the recipe (unless you really like Jello) which is 2 packages of Jello and 1 1/4 cups of boiling water. Be sure to stir it very well (but not so hard that you make a lot of bubbles) and put it in a flat bottom pan with that will allow it to be at least 2 cm thick (a small bread pan works well for a 2-package batch.)
What flavor to use? Yellow (lemon) works well - don't use blue or green, since these will absorb the light (laser safety goggles for red lasers are green). Red will make the beam difficult to see. You can also use plain gelatin (such as Knox brand) but you'll have to figure out the "recipe". Figure 1b shows a portion of a block of gelatin made with two 3 ounce packages of gelatin dessert and 1.25 cups of boiling water. The mixture was poured into a miniature foil bread pan, making a fairly thick block. A sharp cleaver was used to trim a clean edge.
Once solid, the gelatin will stay stiff enough so you don't need to worry about making too much of a mess with your measurements.
2. You will need a piece of paper with two lines drawn as shown in Figure 2. You may want to have several pieces ready in case the first one gets messy.
Figure 2
3. Carefully place the Jello rectangle so that the flattest edge is along the "Jello edge" line. (Figure 3) Direct the laser so that the beam enters the Jello at (above) the point where the normal line meets the surface. You should see the light bend, travel across the Jello, and exit on the other side. (You won't be able to see the beam on the two sides of the Jello because the air normally has no particles to reflect the light. This is where a dusty house comes in handy!)
Figure 3
4. You will need to draw lines as accurately as possible showing the path of the beam from the laser to the surface (the incident beam) and from the surface through the Jello (the refracted beam). To do this, mark the point where the beam leaves the laser and the point where you can see it emerge from the far side of the Jello block. (See Figure 4) If you want, you can continue taking data by moving the laser to a new position and marking the incident and refracted beams. Be sure to label the dots for each trial. Be sure the beam passes through, or just above, the point where the normal line meets the surface.
Figure 4a Mark points where beam leaves laser and where it emerges from block
Figure 4b. Photo of block and laser. Notice that the beam bends toward the normal, and that the beams are not visible in the air on either side of the block. (The red line from the laser to the Jello is drawn in, the red line in the Jello is the laser beam.) Note the three dots marking the end of the laser, the point where the beam enters the jello (the intersection of the surface and normal lines, and the point where the beam exits the jello).
5. Remove the Jello block and draw lines connecting the dots as shown in Figure 5. Measure the angles of incidence and refraction, using your protractor. Be sure to measure these angles from the normal line (that is, the normal line is zero degrees).
Figure 5. Also see figure 4b
6. Calculate the index of refraction of Jello for each trial from the equation:
In this equation, n1 is the index of refraction of air (1.0), q1 is the angle of incidence you measured, q 2 is the angle of refraction you measured, and n2 is the index of refraction of Jello, which is unknown. After you plug in the known angles, solve the equation for the unknown, n2
REPORT- What to turn in:
1. Make a data table showing the measured angles of incidence and refraction and the calculated index of refraction for each trial. Show a sample calculation of index of refraction, clearly indicating the equation you used and how you entered your data into the equation.
2. Find the average value of n for Jello from all of your measurements.
3. Compare your value of the index of refraction of Jello to that of water (1.33). Do you think the value should be higher or lower for Jello? Did you get what you expect?
Other things to do with Jello and a laser, if you feel like playing after the lab is done:
1. Cut a Jello "prism" (triangle) and see how some incident angles lead to total internal reflection at the opposite side. Can you make the beam turn around in the prism by multiple reflections and come out parallel to the direction it entered from?
2. Make a Jello optical "fiber": Cut a long thin ( as thin as possible without breaking) rectangle. Shine a flashlight in one end of the "fiber" and observe the light transmitted to the other end. Bend the "fiber". Does the light still follow the path?
3. Make a y- shaped fiber by carefully cutting the end of the fiber as shown below. What does the light do now?
A