What Causes the Diffraction Pattern When Looking at a Lamp Through a Slit?

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    Diffraction Fraunhofer
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Discussion Overview

The discussion revolves around the observation of a diffraction pattern when viewing a lamp through a slit formed by the halves of a broken razor blade. Participants explore the nature of the diffraction observed, questioning whether it qualifies as Fraunhofer diffraction and the implications of the alignment of the light source relative to the slit.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether the observed diffraction pattern constitutes Fraunhofer diffraction, noting that one condition is that the screen must be far from the slit, while the eyes are close.
  • Another participant asserts that the observed pattern is a Fourier transform of the slit, suggesting that a lens placed correctly could yield an image of the slit instead.
  • A participant references a diffraction grating experiment, questioning if the eyes act as a screen despite being close to the grating, which typically requires the screen to be far for parallel rays.
  • It is proposed that aligning a long light source perpendicular to the slit results in an incoherent source due to varying angles of entry into the slit.

Areas of Agreement / Disagreement

Participants express differing views on the nature of the diffraction pattern and the conditions necessary for Fraunhofer diffraction, indicating that multiple competing views remain without consensus.

Contextual Notes

There are unresolved assumptions regarding the definitions of diffraction types and the conditions under which they apply, particularly concerning the distance of the observer from the slit and the alignment of the light source.

Alan Tam
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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
 
Last edited:
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Alan Tam said:
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.

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!

2. Why does the line filament need to be parallel to the line of slit?

Thanks

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.
 
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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.
 

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