One of the least strange physical phenomena is that light behaves like a wave. Some of the time, at least. (The Younger Pawn asked if she could surf on light, if it was a wave. I said only if she was really really small.)
Physics teachers demonstrate the wave nature of light using the double-slit, or Young’s, experiment. This is quite easy to understand, if you’re at all familiar with the movement of waves upon water: when a wave (of water or light) hits a barrier in which there is a small aperture, you get waves spreading out from the other side of the barrier.
But if you have two apertures (or slits) in the barrier, then the two waves interfere with each other, and you get a well-known pattern. I was going to draw a diagram but as usual, somebody on the internet has already done it for me. Even in three dimensions.
For this to work however, you generally need a coherent light source. Traditionally this involves a pinhole and an incoherent source (which is not what necessarily you put on your kebab after a hard night’s drinking)—hence the S0 in the above diagram—and a lens to refocus the whole shebang. In high school physics lessons they tend to use a laser as the coherent source, and highly expensive engineered twin slit gizmos that the physics teacher gets really upset about if it goes missing.
A couple of weeks ago I was at a film-making workshop and somebody showed a clip that was meant to demonstrate the problems of lighting labs for camera. One of the problematic experiments had a rather large static laser, some smoke to see the beam, a screen and lots of what you might call ‘proper’ equipment. The actual experiment was incidental, but I caught a glimpse of it and was intrigued.
It was, in fact, a variation on the double-slit experiment. The experimenter shone the laser at a wall, and then moved a wire into the beam. Naively, you’d expect the wire to cast a shadow on the wall. But what actually happens is that the wire acts as the partition between two virtual slits: the laser beam is split in two, and simulates a coherent light source shining simultaneously through two slits. And you get the laddering pattern on the wall.
Cool, I thought.
Wait a minute, I thought.
I’ve got a laser. I’ve got several in fact: weedy 1 milliWatt laser pointers with the old F1000 branding. I also, because every kid knows lasers are cool and I’m just a big kid at heart and it was there, have a green 50 mW laser which is a whole heap of fun. I mainly use it for intimidating the dirty pigeons in our garden.
So I raided the hairbrush, held a hair over the end of my laser pointer, and demonstrated the wave nature of light:
The distance from the central spot to the middle of the second maximum is about 4 cm. A human hair is about 100 µm diameter. Now, constructive interference occurs when
and assuming θ is less than about 10°
then 2 x λ = (0.04 x 0.0001)/4 = 1 x 10-6 = 1 µm.
So the wavelength of my laser is about 500 nm. Seeing as it’s rated as 532 nm and I’m a biologist, I’d call that a result.
A couple of years ago I wrote a poem, called Morning. It contains the line,
and sunlight rainbows through Fresnel hair:
which annoyed a certain Stephen Curry, who was upset at the thought of hair diffracting anything on the wavelength of light. I took this as a sign that the good professor has no poetry in his soul, but I now feel scientifically vindicated, in that I have been able to use hair to do this experiment and Fresnel expanded Young’s famous experiment (and came up with an experimentally better version, lasers not being available to him), supporting the wave theory of light. So there.