Description

Below, we present the main idea and equipment instructions of a hands-on activity called Seeing the invisible, that is used at CERN Science Gateway in the framework of 30-45-minute-long workshops for student groups and families.

In this activity, participants conduct a series of experiments to detect hidden structures inside a box. They roll ping pong balls and marbles into the box, observe their scattering behaviour, and infer the shape of the hidden structure. They develop hypotheses by coming up with the best model of what the invisible structure is, practise their scientific reasoning skills, and learn more about the Nature of Science. 

Similarly to mystery boxes, scientists at CERN are aiming to understand the structure of the universe by developing theoretical models and performing complex experiments to confirm or refute their hypotheses. You can learn more about CERN’s research programme here.

Material list

Here, we present the setup used at CERN Science Gateway, which is made using a CNC machine. You can directly access the CNC instructions here. However, there are many different ways to build a comparable setup without a CNC machine. One setup can be used by 1-5 participants. 

• Scattering box made of polyethylene foam (we use Plastazote)
  • CNC machine needed, use file box.f3d
• Different obstacles made of plastic (we use Forex, 5 mm)
  • CNC machine needed, use file obstacles.f3d 
• Aluminium plate (45 cm x 32.5 cm)
• Small whiteboard (30 cm x 40 cm) and whiteboard markers and eraser
• Ping pong balls
• Marbles
• Optional: infrared camera

Instructions

• Prepare all the material needed.
• Take a scattering box, place an easy obstacle inside, hide the obstacle with an aluminium plate, place a whiteboard on top. Place a whiteboard marker and eraser as well as ping pong balls next to the box.
• Let participants roll ping pong balls into the box, observe their scattering behaviour.
• Let participants draw their model of the inferred shape of the obstacle on the whiteboard.
• Encourage participants to repeat and be as precise as possible, before comparing model to obstacle.
• Repeat this with different types of obstacles with increasing difficulty and provide marbles for a higher level of precision.

Ideas for educators

• Start with a group discussion about invisible entities and properties of matter and the need for sensors or detectors to make invisible properties visible. We use two rubber ducks for this demo, one filled with water (i.e. heavier than the other, a difference which we cannot see with our eyes) and one without. We then place them on a balance and it becomes evident that one is heavier than the other. This is a great introductory activity to demonstrate that mass is something we cannot see with our eyes and gets participants thinking that plenty of things in our world are actually invisible.
• Have students work in groups of 2-5 at one scattering box.
• They usually spend 5-10 min with one obstacle.
• Instruct them not to open the box unless you have discussed their scientific model with them or they have explored the whole width of the box and come up with a precise model.
• Increasing difficulty:
  • Level 1: Easy obstacle, which can be detected to high precision with ping pong balls.
  • Level 2: Obstacles that cannot be detected easily with ping pong balls but have smaller gaps that need marbles for detection. Have  students start with pong pong balls to come up with a preliminary model. Then motivate to check their model using smaller balls. A good question to ask is whether they would like to have smaller or larger balls to be more precise before handing out marbles.
  • Level 3: Obstacle that requires using both ping pong balls and marbles to be able to detected as there is a difference in height as well as small gaps.
• At the end, you can give students infrared cameras to study something else that is invisible (i.e. temperature) and that requires (more advanced) detectors.