We look at a lamp at a distance through a slit form the halves of a broken razor blade and a diffraction pattern is observed. My questions are: 1. Is it a Fraunhofer diffraction? Our eyes are close to the slit, but one of the conditions for fraunhofer diffraction to occur is the screen far from the slit. 2. Why does the line filament need to be parallel to the line of slit? Thanks
No, you're not. You're looking at the image of the slit. What happens when light passes through the a slit is that the diffraction pattern that you are seeing is really a fourier transform of the slit itself. But here's the interesting part. If you put a lens after that at the RIGHT spot (i.e. where you would get a focused image), then the lens essentially does an inverse fourier ttransform and you get back the image of the slit! What filament? Zz.
As in the following link of diffraction grating experiment: http://www.practicalphysics.org/go/Experiment_120.html?topic_id=1&collection_id=20 To derive the diffraction gratig formula, we assume the rays emerging from the slits are parallel which requires the screen to be far from the grating. In the experiment, do our eyes act as a screen? But it is close to the grating.
If you have a long light source, and you allign it perpendicular to the slit, then you will end up with an incoherent source, because different parts of the light source will have unequal angle of entry into the slit. Zz.