h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I've read a couple of popular science books recently, because I was in the language stream at school so science didn't really feature for me, so I feel I need to catch up.

I also watched the Brian Cox thing the other night.

What I want to know about is the double slit experiment and to see if I'm witnessing a similar thing. In my bedroom I have outside roller blinds (if you know German houses you'll know the type) which I don't always close completely. That means that in the morning when the light shines through (they are south east facing) I get stripes of light (little blobs actually) as it comes through the gaps in the blinds (where the strips of plastic join together) and hits the opposite wall.

Is that an example of light behaving like waves, or is it me not understanding? (I need to kind of draw what the blinds look like, I think)

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you'd have to imagine that the white space between the black lines isn't there and the light comes only through the gaps (in reality the gaps are about 1cm high and 3cm long)

What you are seeing is an example of diffraction, but it does also demonstrate wave-particle duality to some extent.

When light passes through narrow spaces, it is deflected at the edges of the spaces. When a series of narrow spaces are placed together (like your window blinds), you have photons crossing over as they are 'bent' in different directions. This results in two types of interaction - constructive interference (as both waves are peaking and troughing in-phase) and destructive interference (the opposite). The dark spots you see are the results of destructive interference.

Diffraction itself is most easily explained using the particle theory of light whereas the interference is most easily explained using the wave theory. The interference effects have long been understand from using wave tanks to model light, and it was when this effect resulted from a single photon (rather than lots) that led to duality theory. This led to full quantum theory which states that actually all elementary particles have some wave properties.

B

thanks, B, that was quick.

I think I need to investigate physics further.

The gaps could also be acting as multiple pinhole cameras. The blind on our bedroom window (on the inside) does this. When it is shut there are a number of holes where the strings pass through and these act as pinhole cameras. When the sunrise shines through we get a column of little sun images projected on the wall.

t.

#2 Diffraction itself is most easily explained using the particle theory of light.

No you still need the wave theory to explain light diffraction through a single aperture. The simplest way to see the phenomenon is through the small aperture generated by the first and second finger when placed together (look at a bright light through that aperture). You can calculate the diffraction pattern by splitting the so-called wavefront that passes through the aperture into smaller "wavelets", then calculating how these individual wavelets interfere as a function of angular deviation around the slit. The calculation involves an integration (summing up) over all wavelets across the wavefront. You don't need quantum theory for this - simple wave theory works - it's pre-university level physics.

Bump.

I have finally managed to photograph what I described. I'll upload the pictures to Picasa later.

t.

Looking forward to this....

Here is the link to the Picasa album (hope it works) - http://tinyurl.com/pinholeimages

One shows 2 overlapping images projected onto a door, one the gap in the curtains with the blind behind them and one shows a column of images after I moved the curtains slightly.

t.

Hmm, right, this I'm afraid is not a diffraction interference pattern - at least not in the sense of the double slit experiment.

For mainly two reasons:

1)
A simple basic fact of diffraction seems to have been lost in this thread so far.

Diffraction of waves will only occur around an object or gap *that is of the same magnitude in size* as the wavelength of the wave itself.

Hence if you have 1 meter wavelength waves in a lake then diffraction will only occur through a gap or around a post that is around the same size (i.e. 1 metre ish).

Light has a wavelength that is of the order of nanometres - blue light is about 400 nanometres (i.e 0.000 000 4 m) and even long wavelength visible light (red) is about 700 nm (i.e. 0.000 000 7 m).

In order to diffract it, the gaps in your blinds would have to be *extemely* small - you wouldn't be able to see much in a gap that's only 700 nm wide and it would be a phenomenally gifted venetian blind maker who was able to make blinds to that level of fitting together correctly.

2).
White light from the sun is a blend of lots of different wavelenghts. It is also not coherent (i.e. all waves are not in phase) You can't see a diffraction interference pattern easily from white light like this because all the patterns from each individual wavelegth will interfere with one another and just produced more white light.
Classic double slit experiments were done with light that had been refracted through a prism to isolate a narrower range of wavelengths or more recently, using a laser that gives a very homogeneous wavelength range and coherent phases.


I'm not overly sure of what is causing what you see - maybe the windows are refracting the light first - but this is not a diffraction effect you are seeing in the sense that you think.

I was definitely thinking in my case that the gaps are too big
but it does look pretty - as a non-scientist that's all I can hope for
thanks for that detailed reply though - I'm slowly getting my head around the idea

Oh it's certainly pretty

Incidentally even the glass cannot *diffract* the light.

Atoms are of the order of Angstroms in size (0.000 000 000 1 m) and this is wavelength associated with X-rays. Only X-rays are diffracted by atoms when they shine through a material.
Glass and air will *refract* light, not diffract it.

Incidentally diffraction is a pure wave-effect. Only waves diffract.

The wave-particle duality thing came in when people started observing electron diffraction patterns and they thought - 'hang on, an electron is a particle, how can it be behaving as a wave, like light?'

Glad you stepped in here, Orcus, though not competent to comment, I was thinking that those patterns were more likely to be from the whole sun - edge to edge

... ?

I have to say that I did not claim that what I observed was anything to do with diffraction/interference. I did speculate that what was observed was the pinhole camera effect and later provided an example.

I accept that it was topic drift to some extent but I was trying to suggest what might actually be being observed.

t.

Yes, I should have commented about your post It's much more likely to be the correct answer I think. I did realise you weren't talking about diffraction.

As it happens... standing in the doorway of my shed this morning, sunlight streaming through, making slanting shadows of the jambs and lintel and me...

Where the edges were sharp,
If I moved part of me towards a jamb shadow, it bulged out slightly to meet mine.
If I moved part of me towards the lintel, I bulged out towards it.

It was worth a minute or two playing around.

Comments?

Oh and jambs slight bulge, lintel much more.