On the hairy nature of light

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.

Olaf Wucknitz

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:

wave nature
Photo by J. Rohn

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°

(from Wikipedia)

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.

About rpg

Scientist, poet, gadfly
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25 Responses to On the hairy nature of light

  1. Stephen says:

    Huh? Wha’? I don’t remember that. I deny EVERYTHING!

    To demonstrate the dispersion claimed in your poetry, you need to repeat the experiment with the red laser and show different angles of diffraction from the green one.

  2. rpg says:

    I thank the reviewer for his comments.

    We predict that a red laser should display an increase in spacing of 20%, if the poetry hypothesis holds. There are experimental restrictions, but we are working to overcome them and will publish our findings in a follow-up paper.

  3. cromercrox says:

    = likes =

  4. Tideliar says:

    I am so madly in love with you it hurts sometimes.

    Awesome. Simply awesome.

  5. Eva says:

    So the hair is perpendicular to the ladder pattern? That is so cool!

  6. rpg says:

    You’re too lovely.

    Yup, the hair is vertical and the ladder comes out horizontal. Classic.

  7. Alejandro says:

    I miss a phase diagram…..say…

  8. David McGloin says:

    You should see what you can do if you put a hair *inside* a laser cavity – all sorts of lovely patterns are possible. Not so easy with a laser pointer mind…

  9. rpg says:

    Ooo. Now that has potential (I have a number of the things, I’m sure donating one or two to science would be acceptable).

  10. Bob O'H says:

    Hey, how do we know that photo isn’t actually of some bacteria madly expressing GFP? I’m sure a bit of signalling with cAMP could create the same effect.

  11. rpg says:

    Oh… I have some GFP plasmid in a box somewhere…

    you bastard, Bob. Now look what you’ve done.

  12. Awesome! And I just use my laser pointers to annoy the cats…

    Joke I just made up:

    Student: “Are lasers dangerous?”

    Physics professor: “I’ve no eye, dear”.


  13. Steve Caplan says:

    Great post, and superb photo!

    Reminds me of a story told by a colleague about his, well er, rather elderly post-doctoral mentor. She was apparently very highly regarded and a frequently invited speaker. However, she never quite did get the hang of using a laser pointer, and when she spoke and brandished her pointer, the audience would collectively duck and put up blocking arms to avoid being blinded…

  14. steffi suhr says:

    Very cool!

    But aren’t you a couple of months late with this post, considering “the laser” turned 50 last year?

  15. rpg says:

    Oh dear, Cath. Do not look into laser with remaining eye.

    Although the green beast won’t hurt you, it can leave a bit of an afterglow. I’m not usually allowed to use it in the house and I have to supervise the girls real close when they nag me for a play. I did find a 200mW one on sale yesterday, which can apparently light matches. Tempted.

    Steffi, cool science doesn’t need an occasion.

    Oh, and the next phase of the experiment went very well. I have to find a ‘representative’ photo though.

  16. Brian Clegg says:

    I can only quote Richard Feynman (argue with him if you dare) when describing how the behaviour of light can be explained by QED:

    “I want to emphasize that light comes in this form – particles. It is very important to know that light behaves like particles, especially for those of you who have gone to school, where you were probably told something about light behaving like waves. I’m telling you the way it does behave – like particles.”

  17. Jonathan says:

    The good folks at the physics department in York once regaled me with stories about how they used to tune the gas mix in Helium/Neon lasers, back in the ‘good old days.’ The procedure involved looking straight down the beam axis as you bled in one of the gases, with the flash tube running. When you saw the colour start to change you (a.) turned off the gas valve and (b.) got your head out of the way. In approximately that order.

    I’ve no idea if the story is true, but I like it.

    The film to which Richard refers is here:

    It’s an instructional film for school teachers, and it’s a little more plodding than I’d prefer. But if you want more hot laser diffraction action, there you go.

  18. rpg says:

    Yow. Thanks for the link, Jonathan.

    I should really have made my own film, but my experimental setup isn’t really appropriate to that (the laser support is actually my camera tripod…). The next part of the experiment is coming soon–and it fit well with the theory (see previous comment).

  19. ricardipus says:

    That may be the most awesomesauce thing I’ve read all year. Maybe even last year too. I suspect you’re faking the math though.

    *thinks – I also have an F1000 laser pointer, and some hair
    *checks – and it’s red! The laser, not the hair.

  20. ricardipus says:

    P.S. Cath – that joke is 100% excellent.

  21. Steve Caplan says:

    @Cath and Ricardipus,

    Cath’s joke puts her as a frontrunner for “Science Joke of the Week”…

  22. ricardipus says:

    P.P.S. I can now confirm that a red F1000 laser pointer and one of my own hairs works beautifully. Photographic evidence will be lacking, until I grow a few more hands. The cheapo red pointer from a CHI conference, however, did not – I think because its lens doesn’t make a very tight beam.

    I can also confirm that said hair(s) came out of my scalp with distressing ease.

    Perhaps fittingly, the first half of the reCAPTCHA for this comment was the compound word “Artforum”. 🙂

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