This green sheet is used to see the invisible force fields of magnets. It is made of nickel flakes floating in tiny fluid-filled capsules.
Nickel is one of only three elements that are naturally ferromagnetic. Since these nickel particles can move freely within the thin layer, they are influenced by nearby magnetic fields and orient themselves accordingly.
When placed directly over a magnet, the light areas show where the magnetic field is parallel to the sheet, so the flat sides of the nickel particles are reflecting light. The dark areas are where the particles are aligned edge-on, meaning the field is oriented perpendicular to the sheet.
This helps identify the poles of any magnet, or find components with magnets like speakers and hard drives. Try to see how many magnets are in a laptop or phone.
Since this film shows a two-dimensional cross-section, moving it toward or away from the magnet helps visualize how the field changes in 3D space.
This material gives humans the ability to see a force that would usually be invisible to us.