[❔ other question] Snell's law not followed by light

[MaxDevas]

I try to replicate some experiments in Mitsuba. I wanted to try to see snell's law of refraction But the light does not follow the path it is expected to take.

I put a spotlight, a cube with bsdf null and a isotropic medium. And a cube with bk7 (The cube is the default blender cube i exported as a .obj file)

I launch Mitsuba in scalar_rgb mode, i tried with scalar_spectral

But i tried another experiment, where the sensor looks at a picture which is under a glasscube. and in this case it seems to be fine

Here are the two scenes i tried. refractionzip.zip

Is there something i didn't understand in the computation of the ray? Is there something I did wrong in the scene?

Thank you in advance.