8B10.50 - Sunspots on an Overhead
To demonstrate why sunspots appear dark on the surface of the sun.
Place a 500W filament light bulb on its side using a 2.25" tall board on top of an overhead projector. Focus the overhead onto the filaments of the bulb and aim it to hit the white projection screen in the classroom. Turn the light bulb on so that it has a visible orange-ish glow. When the overhead projector is turned on, the hot filament will appear black in comparison to the output of the overhead.
Turn off the room lights and turn on the filament bulb to let the students see the orange-ish glow of the filament. Then flip on the overhead projector and observe the orange-ish glowing filament appear black in comparison.
The temperature of sunspots are typically around 4500K, producing an orange-ish appearance. The photosphere of the sun is typically 6000K, producing a yellow-ish appearance. In comparison, the sunspots appear black against the backdrop of the hotter and brighter photosphere surrounding the sunspots.