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Many spells shed light as one of their effects, such as the Sunbeam spell, which says:

For the Duration, a mote of brilliant radiance shines in your hand. It sheds bright light in a 30-foot radius and dim light for an additional 30 feet. The light is sunlight.

We know that spell effects are blocked by total cover (PHB, p.204):

A spell’s effect expands in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn’t included in the spell’s area. To block one of these imaginary lines, an obstruction must provide total cover.

And that transparent obstacles still qualify as total cover (citation needed, but I'm pretty sure about this one).

Thus it seems to be the case that a glass window (or other transparent obstruction) can block magical light. However, this seems odd when you consider non-magical light can obviously pass through it fine.

So: can light from a spell like Sunbeam pass through a window, or not?

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  • \$\begingroup\$ While Jeremy Crawford's tweets are no longer authoritative, we do have this answer from him confirming that solid obstacles provide total cover, regardless of material. Still looking for a rules based ruling here. \$\endgroup\$ – Mwr247 Dec 13 '19 at 19:29
  • \$\begingroup\$ The answer may change depending on what is the cover, can you specify? \$\endgroup\$ – Bruno Souza Dec 13 '19 at 19:29
  • \$\begingroup\$ @BrunoSouza Actually, the crux of the issue as the rules do do not distinguish the nature of the full cover, just the existence. \$\endgroup\$ – T.J.L. Dec 13 '19 at 19:40