The Shadow Wall exhibit is about light absorption. Light may be both reflected and absorbed at the surface of a material. The material will radiate away this absorbed energy continually, often as heat or infrared radiation which our eye cannot detect. For example, stones on a beach absorb sunlight and heat up.
Some materials are phosphorescent which means they can absorb light and then re-radiate it.
That's the kind of material with which we've coated our Shadow Wall. By standing in front of the phosphorescent wall, when the flash goes off, your body blocks the light from the flash. The light that reaches the wall makes the wall glow. The area that was blocked by your body looks like a shadow on the wall.