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The Huygens–Fresnel principle states that every point on a wavefront is itself the source of spherical wavelets, and the secondary wavelets emanating from different points mutually interfere.The sum of these spherical wavelets forms the wavefront. So, why don't the points on a wavefront generate secondary wavelets when a wave propagates in empty space and cause diffraction? Why does diffraction occur only when a light wave passes through an opaque edge or a slit?

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    $\begingroup$ Re, "So, why don't the points on a wavefront generate secondary wavelets when a wave propagates in empty space...?" But that's the whole point, they do generate secondary wavelets. That's what "wave propagation" is. The sum of all of those "secondary wavelets" (and tertiary wavelets, etc.) in free space looks like an expanding spherical wave front (assuming a point source, some finite distance away.) $\endgroup$ – Solomon Slow Aug 24 at 18:27
  • $\begingroup$ Then why doesn't diffraction occur in free space but occurs only when the light wave passes through an opaque edge or slit? $\endgroup$ – Ztz Aug 24 at 18:31
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    $\begingroup$ Because "diffaction" is the name for what happens when part of that expanding spherical wave front is obstructed. The wavelets that make it past the obstruction no longer add up to the same source-centered spherical front as before because they have no neighbors in the "shadow" of the obstruction. Since each point acts like a new source, the waves originating from the edge of the shadow propagate into the shadow. $\endgroup$ – Solomon Slow Aug 24 at 18:35
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    $\begingroup$ An opaque edge is not required. For example, a lens can be understood to work by diffraction of the waves emerging from a transparent material of varying thickness. Also a hologram works by diffraction, but can be made entirely from transparent materials. $\endgroup$ – The Photon Aug 24 at 18:38
  • $\begingroup$ So, the whole point for diffraction is just to block some part of the wave and let some part pass through so that the interference of the secondary waves create a different effect than while propagating without obstruction? $\endgroup$ – Ztz Aug 24 at 18:43
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So, why don't the points on a wavefront generate secondary wavelets when a wave propagates in empty space and cause diffraction?

They do. As Solomon points out in comments, free space propagation can be described in exactly these terms.

Why does diffraction occur only when a light wave passes through an opaque edge or a slit?

It doesn't. It can also occur when one part of the wave experiences a time delay or phase shift.

For example, holograms and fresnel lenses work by diffraction through phase shifting materials rather than opaque materials.

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  • $\begingroup$ Then how do rays remain parallel in empty space? Why don't they spread in all direction by creating secondary wavelets? When a laser produces a narrow beam of light, why doesn't light spread in all directions by creating secondary wavelets? $\endgroup$ – Ztz Aug 25 at 4:37
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    $\begingroup$ @Ztz, rays are only an approximation to wave behavior. They only remain parallel when they represent part of a plane wave of infinite transverse extent. In that case if one ray "diverges" then the other rays around it do equally, so that the net effect is that the plane wave continues without diminishing. Any beam of light with non-infinite diameter will in fact diverge due to diffraction. $\endgroup$ – The Photon Aug 25 at 4:57
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The definition of diffraction from wiki:

Diffraction refers to various phenomena that occur when a wave encounters an obstacle or a slit. It is defined as the bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture.

So for your question

Why does diffraction occur only when a light wave passes through an opaque edge or a slit?

The answer is the definition of diffraction requires an edge or slit

Note that the definition is more narrow than simply interference or beam forming.

'diffraction' from dictionary.com:

  1. the phenomenon exhibited by wave fronts that, passing the edge of an opaque body, are modulated, thereby causing a redistribution of energy within the front: it is detectable in light waves by the presence of a pattern of closely spaced dark and light bands (diffraction pattern ) at the edge of a shadow.
  2. the bending of waves, especially sound and light waves, around obstacles in their path.
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