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I have a scene where the sun rays should be reflected from a mirror to create this sort of effect:

sun ray

The problem below was resolved, scroll down to EDIT.

However, I can't get any light reflected from any glossy surface. I set up a simple scene to test it ans here is my result.

scene

And here is rendering.

mirror

But the cube stays dark.

cube

I will be very happy if somebody could help me with this.

Blender file:

EDIT:

enter image description here

I fixed the problem with the test file by adjusting light position and size (and maybe something else). For that I added a ground plane and was adjusting the parameters until I got the shadows right.

Increasing the roughness parameter, suggested by Duane Dibbley makes rendering faster and less noisy. However, then it is not a mirror any more. When the roughness is zero, rendering takes a lot of time and doesn't look like it is going to finish.

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    $\begingroup$ Could you show your Light Path settings too? $\endgroup$ – Carlo Aug 5 '16 at 22:22
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    $\begingroup$ @Carlo, I added Blender file, I think it is better :) $\endgroup$ – Noidea Aug 5 '16 at 22:30
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    $\begingroup$ If I understand this correctly, what you are trying to do is currently not possible using cycles. See this question: Reflecting direct light on glossy surfaces $\endgroup$ – cegaton Aug 8 '16 at 17:39
  • $\begingroup$ related: blender.stackexchange.com/questions/24382/… $\endgroup$ – cegaton Aug 8 '16 at 17:39
  • $\begingroup$ @cegaton thanks, that is the thing I was looking for. I got it the example working now. In a real scene I have a mirror, but reflection is full of fireflies, so it will take ages to render. When I change the roughness then mirror is not sharp any more (and the reflection). Maybe I'll try that patch some time later on... $\endgroup$ – Noidea Aug 8 '16 at 18:52
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The roughness setting is insanely low, which makes the reflected light beam very narrow. If you increase the roughness, the light will be spread in a wider beam. This will, of course, also make the reflection of Suzanne somewhat blurry. In the image I've included, I have it set to .1, but I also tried .01 (10 times the .001 you have), and that made the interior of the cube visibly brighter.

enter image description here

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  • $\begingroup$ Thanks a lot! Did you also change something else? Like light brightness or something? Because my rendering looks very different - see edit. $\endgroup$ – Noidea Aug 5 '16 at 22:54
  • $\begingroup$ I did play with the strength and size of the sun lamp, but I'm almost positive I changed it back. $\endgroup$ – Duane Dibbley Aug 5 '16 at 23:11
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Cause of the Cube Being Dark

The caustics are present in the preview render, the inside of the cube is lit by the light: Test it by disabling caustics, the inside of the cube gets darker. Note, that you only get a small portion of ligh, because the mirror is small, and only a small part of the light is visible from the cube. If you increase the size if the mirror, or decrease the size of the lamp, the inside of the cube gets brighter.

Small Mirror Big Lamp

Small Mirror Big Lamp

Big Mirror Big Lamp

The big mirror covered the scene, I have hidden it only for the camera.

Big Mirror Big Lamp

Small Mirror Small Lamp

Direction of lamp had to be adjusted.

Small Mirror Small Lamp

It worked for me with a brand-new version for Windows, that I just downloaded on my work computer. In case, it does not work for you, try to open your Blend with a fresh copy of Blender.

Reducing Noise

Getting rid of some of the noise can be done using the Properties Panel / Render / Ligth Path / Filter Glossy property. For caustics, the mirror will behave like a rough surface, but for the camera, it will still look like a mirror. This is exactly the case, why it was introduced. Have a look at the documentation page for reducing noise.

Small Lamp High Filter Glossy

I did not perfectly catch the same direction as before, but the noise reduction is somewhat visible.

Small Lamp High Filter Glossy

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