1. The problem statement, all variables and given/known data The photograph shows a device called a radiometer. The four regular panels are black on one side and shiny like a mirror on the other side. In bright light, the panel arrangement spins around in a direction from the black side of a panel toward the shiny side. Do photon collisions with both sides of the panels cause the observed spinning? 2. Relevant equations - 3. The attempt at a solution I thought a radiometer could only spin by shining light (photons) on it. And since the shiny side reflects more light, light shining on this side would make it spin in a direction from shiny towards the black side. But here it says it spins from the black panel towards the shiny side? How is that possible? What is it then that causes this spin?