If you look through a standard net curtain that hangs over a window, in your home, and observe the lights in your room there is no special effect. They appear as normal.
Now try this experiment: get a really small light source, such as the LED light on your iPhone. Put this directly behind the curtain, and again nothing special happens. It appears as normal.
Now gradually move the curtain away from your iPhone. At distance of a about one meter from the light source, you'll begin to see what looks like a diffraction pattern.
Can you explain what is going on? Can you quantitatively predict the distance between the curtain and the source you need for the pattern to appear?
Nice question for an exam in optics in a general physics course. Add: "How length of coherence affects the distance?" Or mention Ernst Abbe somewhere.
Sorry I don't have an answer, but this brought to mind another strange phenomenon I experienced a year or so ago.
One very dark and clear night I looked up at the stars above some 20 meter tall white pine trees. Several stars were visible *through* the pine needles at the very top of those trees. If I moved my head a few inches to the left or right the *apparent* position of the stars would flicker but move smoothly ... then suddenly jump. At that critical-point, a slight movement of my head back and forth would cause the star's apparent position to jump back-and-forth a noticeable fraction of a degree. My guess was that there was some kind of diffraction occurring through needles on branches at slightly different distances or between needles clustered with different spacing, but it was too dark to make out the structure of the tree at that height. Any thoughts?
A simple view to imagine is to think on the diffraction pattern as lying on the point source (LED) plane (details in diffraction books, as Goodman, Introduction to Fourier Optics). For a grating (net) the pattern is a set of points, with spacing L*lambda/P, L being the distance point-net and P the period of the net (small angles assumed). As the distance is increased the distance between the points increases. when the distance is larger than LED size you see the multiple images. The maximum perceived separation will be for the larger possible distance, infinity, and you will see the multiple images (orders of diffraction) separated by an angle sin(alpha)=lambda/P
I agree, this has nothing to do with diffraction. Diffraction always depends on the wavelength. This is why a grating has a colourful refection. In your picture the pattern is just white. Also, the spacing of the holes in the net and the fluctuations therein are enormous compared to the wave-length of light.
The explanation is much more mundane: The fibre of the netting is probably fibre glass or some plastic. The surface is very smooth and acts like a tiny cylindrical mirror, which scatters (reflects) light in a plane orthogonal to the fibre direction.
Spatial coherence = point source. And we have Fraunhoffer diffraction because we focus on the retina (forget about all books, saying you must collimate previously light, to put at the focal plane after the diffracting object). You could even see 'almost' white light fringes, but some spectral colouring appears if you can look at the whole pattern.
- Badges
- Science topic