Description

Dark matter is one of the most intriguing scientific mysteries in modern physics. Large galaxy clusters contain both dark matter and normal matter, and the immense gravity from all this material warps the surrounding space. This warping distorts and magnifies the light from objects located behind the cluster—a phenomenon known as gravitational lensing.

This activity offers a playful and accessible way to introduce the concept of gravitational lensing—using jelly! Students build their own "jelly lenses" to simulate how light bends around massive objects, just as it does around dark matter in space. 

At CERN, we use powerful particle accelerators to search for hypothetical particles that may make up dark matter. You can learn more about dark matter and the tools CERN uses to search for it here.

Material List

You will need the following to create and experiment with jelly lenses:

• Laser pointer (safe to use)
• Food colouring in the colour of your laser pointer
• 10 g gelatin
• 500 ml water
• 50 g sugar (or sweetener)
• Small pot and heater
• Plastic or glass boxes
• Knife
• Hot water
• A surface you can clean easily such as a cutting board
• Sheet of white paper
• Optional: Flavour (e.g. lemon juice or raspberry)
• Optional: UV lamp
• Optional: Tonic water (to make the jelly fluorescent under UV)

Instructions

You can watch a tutorial of the lens-making in this Youtube video.

Instructions to build the jelly lenses

• Check the instructions on your gelatin package but use at least twice the recommended amount. We suggest 10 g of gelatin with 500 ml liquid.
• Add gelatin, water and sugar to a small pot.
• Add a few drops of food colouring that match the colour of your laser pointer. Be careful not to add too much colour, as it could block the laser beam.
• Optional: Add flavour such as lemon juice or raspberry for taste.
• Stir gently and heat the mixture until the gelatin and sugar dissolve completely—there is no need to pre-soak the gelatin.
• Pour the mixture into plastic or glass containers.
• Let the jelly solidify in a fridge for a few hours, ideally overnight.
• Ensure the jelly height is around 2 or 3 cm for better stability.

Instructions to operate the experiment

• Prepare a clean workspace. Use a cutting board and if needed apply a thin layer of oil or cooking spray to prevent the jelly from sticking to the surface.
• To release the jelly from the containers, place them briefly in hot water.
• Heat the knife in hot water before cutting the jelly.
• Cut the jelly into triangular shapes, preferably with edges of 5 to 10 cm.
• Fold a sheet of white paper twice lengthwise to serve as a screen.
• Place the triangular jelly shape with the two equal sides facing the screen and the third side facing the laser pointer.
• Switch on the laser pointer and direct the beam through jelly lens.
• Dim the lights to improve the visibility of the laser beam path.
• Observe how the laser beam bends while passing through the jelly, mimicking gravitational lensing.

Ideas for Educators

• Explore variations of the experiment, such as changing the shape of the jelly lenses.
• Add a twist by using tonic water. Under UV light, the jelly will fluoresce faintly in blue because of the quinine content of tonic water, perfect for Halloween!
• For more information about how to use this activity in your classroom and which students' conceptions you should be aware of check out this open-access article: Woithe, J., & Kersting, M. (2021). Bend it like dark matter! Physics Education, 56(3), 035011. You can read the article here.

Related Resources

• Explore how scientists infer the existence of unseen objects by observing indirect effects (just like how we detect dark matter by observing the bending of light) with this 3D-printable hands-on activity called the Mystery Boxes. You can access the activity here.