- Preparation & Notes
- Downloadable Files
Optics with Jello
Lenses and magnifying glasses are instruments we use to bend light for some special purpose. The process of bending light (or other waves) is known as REFRACTION. The lens of the eye focuses light on the retina. Eyeglasses have lenses that correct for when the eye does not focus light properly on the retina.
Materials with a refractive index that differ from air can bend light. In this exercise, students will exploit this property in gelatine and make (i) a magnifying lens, (ii) a convex or converging lens, and a (iii) concave or diverging lens, all out of jello. Red laser pointers will be used to trace the path of light through the jello.
(i) Jello Magnifier
(ii) Convex Lens
Students can create new shapes and observe the reflection and refraction of laser light. This activity takes some preparation to make the gelatine and can make a mess but students LOVE playing with jiggly jello. The images of the laser path through their lenses are dramatic.
Jello magnifier on top of a CD cover with message in tiny letters underneath
Red lasers shining through a convex jello lens
Red lasers shining through concave jello lens
"D" Shape convex lens can be used to show internal reflection
Jell-O lenses and magnifying glasses
- Knox unflavored gelatine or Red Sugar Free Jell-O
- boiling water
- red food coloring (if using unflavored)
- 8”x 11” lasagne pan
- 1 popsicle stick
- bubble gum machine toy container
- wax paper, Vaseline or oil
- smooth knife
- cookie cutter "D" shape
Ray Fan - Parallel Laser Rays
- 2 – 5 red laser pointers
- small piece of cardboard
- scotch tape
- block for turning on laser pointers
Activity 1. Observation of refraction
- Shine a laser pointer at the rectangular jello and observe refraction, the bending of the laser beam when it hits the jello at an angle.
Is the beam bent the same at every angle?
Activity 2. Convex and concave lenses.
- Shine the mounted ray fan through the convex jello lens. Look to see the beams converge after exiting the jello. It may help to aim the light low on the surface. Can you see the focal point?
- Repeat this process with the concave jello form. Can you see the laser beams diverging?
Activity 3. Magnifying jello
- Take the bubble shaped jello and slide it over printed material with clear plastic covering it. The letters will be bigger when looking through the jello. You may be able to read a tiny secret message.
Activity 4. Experiment with your own shape
- Use the mold or plastic knife to create a shape of your own to experiment.
- Shine a laser pointer through it. Observe refraction and reflection of the light.
Preparation of gelatine for forms
- Line lasagne pan with wax paper. The pan can hold 8-10 cups of gelatine.
- Boil water and combine 8-9 **unflavored gelatine packets with 4 cups of water. (1 packet in 0.75 cups water will produce a firm gelatine) Stir until clear. Add 4 cups of cold water, a couple drops of red food coloring and pour into lasagne pan.
- Chill for 2-4 hours
- Use knife to cut lens shapes into the gelatine. Useful shapes include concave, convex, D-shaped lenses and a simple rectangle.
Suggestion: Prepare 2 pans, one for pre-cutting and one for student experimentation.
Preparation of magnifying lenses
- Combine 2/3 cup boiling water with 1 packet from unflavored gelatine. Stir until clear and bring to ¾ cup with cold water.
- Take toy bubble lid, poke a hole in top and cover with duct tape.
- Lightly grease the lid with oil or Vaseline
- Pour the gelatine into the bubble lids and refrigerate for 2 hours.
- Getting the lenses out is tricky. Remove tape and run cold water over hole. Pour water over the top of the gelatine. Gently squeeze the bubble and let water run down the gap between the side and the jello.
- Try using a popsicle stick in this gap and tilt the bubble sideways. The jello should slide out in one piece.
Preparation of Red Laser Raytracer
- Take 5 red laser pointers and lay them side-by-side with the on-buttons facing up on a small piece of cardboard.
- Use tape to secure laser pointers to the cardboard parallel to each other.
- The wooden block can be used to turn the laser pointers on simoultaneously by pressing down across all 5.
Sources and Additional Reading
A nice tutorial by Cynthia Furse at the University of Utah
A description of Laser Jello at the Exploratorium in San Franciso
Blackboard Optics with great images of refraction with lenses.
** Unflavored gelatine is not as messy as flavored Jello, but the students enjoy playing & eating with flavored Jello. Sugar-free is less sticky, easier to handle & dissolve than regular flavored jello. Red gelatine reflects the red laser pointer and will have a brighter image in the gelatine than other colors.