Sunday, 4 January 2009

Wave nature of light demonstrated by hand


You can do this at home as long as you have a source of light, two fingers and a working eye.

Hold up your hand a couple of inches in front of your face with two fingers very close together.

Look at a more distant light source (not the sun) or light coloured object. I used my computer screen.

Now bring your hand in the way so that you can see part of the light object through the gap.

Try to keep focussed on the more distant object rather than your fingers.

You should be able to see some dark lines running parallel with the edges of your fingers.

These are interference patterns!

You have just re-created the same effect that is seen in the most famous physics experiment of them all - the double slit experiment - well kind of.

How cool is that?

Here is a picture- see for your self.

You can just about make them out on the picture - they are much clearer in reality.

Good luck.

Next week we will be attempting to achieve fusion using three sheets of toilet paper, a paperclip and a bicycle pump.


  1. Thank you! That is cool! I still remember my amazement in H.S. physics class to learn that light's behavior required two competing models, the wave and the particle theories. I am even more amazed that 40 years later, nothing has much changed in that regard. I find it fascinating that so many fundamental mysteries (i.e. gravity, and the anomalous behavior of light) remain after all we have learned. Thanks for the reminder!

    From Cliff - A Christian friend of mine from the States - blogger commenting seems to be playing up!

  2. I just assumed it was blurred fingers because you weren't looking at the fingers but at the light source across the room.

  3. madge waves : )

  4. @Psiloiordinary - I know this is a bit late, since you posted in January, but you might want to check out Bohmian Mechanics. It describes and predicts everything that standard quantum mechanics does, but does not require two competing models. Much has changed in 40 years.

  5. I´m sorry to contradict, but the effect you´d be observing is diffraction, not interference, since like you mentioned an interference pattern happens when you have at least 2 different slits.

  6. Hi Anon,

    You are right - and so am I - I think.

    We are seeing the interference pattern from the diffraction around both of the fingers.

    Try a simulation here;



  7. I'm about to be tested on this tomorrow in my quantum analysis class.

    How lovely.

  8. Fingerprints.

    The "effect" goes away when you use pieces of paper to make the slit.

  9. I don't want to seem dismissive, but, no it doesn't.