When I first built Dr. Altman’s Amazing Laser Music Can, I was captivated. This is true science! Gorgeous laser light that was unimaginable before the development of quantum physics. Patterns generated by varying partial derivatives of the surface of a rigid plate on a vibrating membrane – and you thought calculus was boring!
I loved it, and after playing with it a bit, I wanted more. I read up on laser light shows used in raves and clubs, and found they relied on on mirrors attached to finely controlled motors, directed by panels of digitized electronics. First off, I can’t afford that. Secondly, it seemed rather convoluted that audio had to be digitized, fed into a program, translated to thousands of tiny precise mirror movements, and then you could finally see an effect – when I could do something similar, albeit not controlled to quite as fine of a degree, with a balloon and a mirror.
So what to do? Well, it seemed that the only affordable approach that was relatively simple to build would have to be similar to the Five Dollar Laser Show approach – music causes a mirror to vibrate, a laser bounces off of it, and a pattern is generated. I knew I wanted a brighter laser, and I wanted lots of those patterns.
Splitting one strong beam into lots of smaller parts seemed to be more practical than buying 200 lasers that would all bounce off one mirror – and something called a “diffraction grating” fits the bill perfectly. This is another example of the magic of physics – it’s sort of like a window screen fixed in glass that’s so incredibly fine you can see through it and not notice the individual wires. They’re so tiny that the laser goes through all these little holes, and the wave-like nature of light causes each of these little parts of the laser light to “interfere” with each other.
What do I mean by “interfere”? Well, let’s think about a wave property we’re familiar with – waves in water. If we drop one rock in water, we can see nice round ripples moving away from the rock – there’s no interference occurring here. If we drop two rocks at the same time, the situation changes. At first, we get normal circular ripples moving away from each rock, but then the ripples hit each other – and start to interfere with each other like we see in the video above. In places where the peak of two ripples meet, we get a ripple twice as big. In a place where the peak of one ripple and the trough of another ripple meet, we get no ripple at all!
Now obviously this diffraction grating is more complicated than ripples in water – but something similar happens on a very small scale. Laser light, which like all light has a wave-like nature, comes out of all the little holes in our diffraction grating and interferes in a very regular and predictable way, giving us all these wonderfully split beams. Remember – it’s not the little holes in the diffraction grating alone that split up the light, it’s the splitting and how light interacts with itself, the “interference”.
Enough theory, here’s the concept. A powerful laser reflects off a mirror mounted on a speaker, and then goes through a diffraction grating. This will create not just one laser pattern based on the vibration of the speaker, but hundreds. Sweet.
There are a huge variety of ways to modify this basic concept with more mirrors, more diffraction gratings, rotating mirrors/gratings, extra lasers and speakers – but I figured that I should probably build the first version to be as simple as possible. If engineering has taught me anything, it’s that the gap between design and execution is a bit more involved than we like to admit.
Now that you have a handle on the concept and design – on to what you need to build it!