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In diffraction from a single slit, we learn that the angular width of the central maxima, is given by $2\sin^{-1}\frac \lambda d$. For $d\approx \lambda$, the incoming wavefront should be spread to almost all directions.

Then, when a beam of light passes through a transparent material such as glass, the inter-atomic spacing in the material is almost comparable to $\lambda$, then why does not the incoming light be spread due to diffraction in the entire $2\pi$ steradians, i.e angular width of $\pi$ radians in all directions? Why is it still transmitted approximately as a beam?

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The wavelength of visible light is about 500nm and the interatomic spacing in silica glass is about 0.15nm so there is a factor of around 3,000 difference between $\lambda$ and $d$. That's why light isn't diffracted by the atoms in glass.

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  • $\begingroup$ That means that $d$<$\lambda$, does that not violate the condition $\sin(\theta)\leq 1$? If it does, how do we find out the angular width in which it diffracts light? And for arguments' sake, what if there is a transparent material with $d\approx \lambda$, will it diffract light in all angles? $\endgroup$
    – user4059
    Jan 15 '14 at 10:42

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