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As the title says... I have a transparent (glass or full transmission in the principled shader) plane, but I see the reflection of my area-light although the material is not reflective. The only way it disappears is when I set the IOR on the plane to 1, which defeats the purpose of my setup. Any ideas? Bug? why

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    $\begingroup$ Glass is reflective. $\endgroup$
    – user1853
    Jun 4 '19 at 14:53
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    $\begingroup$ Everything works like expected. You have set your glass to be perfectly reflective on the surface. That is what Roughness = 0 means. It is a perfect mirror surface with a glass volume behaviour. $\endgroup$
    – BK.
    Jun 4 '19 at 17:22
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According to the manual:

The Glass BSDF node is used to add a Glass-like shader mixing refraction and reflection at grazing angles. Glass BSDF

That's why you see reflections with this kind of shader.

If you don't need reflections on the object - use Refraction BSDF

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  • $\begingroup$ Accepted...although I don't get why it happens with the Principled BSDF with Transmission Full and Specular off.... $\endgroup$
    – knekke
    Jun 5 '19 at 6:32
  • $\begingroup$ Under the hood of Principled BSDF just mixing of other shaders. Example from Principled BSDF manual > Transmission is Mix between fully opaque surface at zero and fully glass like transmission at one. $\endgroup$
    – RUben
    Jun 5 '19 at 8:26
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All (smooth) surfaces in the real world become reflective at low angles of incidence, Eevee and cycles (with Principled shader) reproduce this, but Blender Render didn't.

As RUben says using a refraction BSDF will refract the light as if it entered a material of the same density of glass, without simulating the reflections that you would get when shining a light through real glass.

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