This is my big disco ball. It’s about the size of a football. I’ve got a little motor thing for turning it too. Unfortunately I don’t have a suitable light for shining on it. Not just any light will do. I need a spot light really. A normal light bulb doesn’t work very well for two reasons:

• Firstly it lights up the whole room, so the spots of light on the wall dont look so impressive because the wall is already lit. We do have some directional lamp things which work better, but in general a normal light bulb is not good for dingy disco darkness
• Secondly a normal bulb creates rays of light which diverge as they radiate outwards. If the bulb is a couple of metres away from the disco ball then you might think that the divergence would be negligable, but no! Unless the light rays are completely parallel, you’re wasting your time. The reflections form faint fuzzy blobs on the wall, rather than impressive well focused spots.

…but maybe I’m wrong about the whole parallel thing. Obviously the correct equipment for the job is a spot light. It doesn’t take a PHD in disco-lightology to tell you that. But what’s so special about a spot light? Do these have parallel rays? No they dont, because otherwise they wouldn’t be able to make big 2 metre wide spots, big enough for a ballerina to stand in. But the light is focused, so that you can put silly shaped filters on them (or dog shaped shadows with your hand). A projector is the same. A projector would be good for disco balls too.

The staircase trick

I have had some success with my disco ball. If I turn off all the lights in the house except for the light on the top floor landing. This particular lighting configuration doesn’t occur often, so it was quite an exciting discovery when I noticed the nice well focused spots on the darkened living room ceiling. Light beams were travelling 3 stories down through the narrow gap in the middle of the staircase and striking one side of my disco ball where it hangs above our sofa.

Direct Sunlight

The most impressive thing to try with a disco ball is direct sunlight. I might organise a disco party on a sunny day to make use of this. Sunlight creates perfectly focussed bright spots. Roll the ball from side to side in a patch of sunlight near the window, and it feels like the whole room is rolling!

The circular square disco ball spot phenomenon

Here’s a physics question for you. Given the correctly focused lighting conditions discribed above, why is it that the spots on the wall are circular? I dont just mean blobby roughly circular shapes. They are perfect crisply defined circles. Reflections at about 10-15cm from the disco ball are square shaped, matching the shape of the mirrors exactly, as I would expect. So how come the long distance spots are circles? Surely this is an important question which science must have an answer to. It’s right up there with “how did the universe begin?”