In a Shadowrun 4e game I play in, an adept with the Astral Perception power wanted to sit in a bar and astrally spy on something behind the window. SR4A p.191 states :
Non-magical and non-living objects have only gray, lackluster shadows rather than auras, but pick up impressions from being in contact with living auras.
It is unclear about transparent materials such as glass, or reflective surfaces, how does this work ?
As explained in Street Magic,
An astral shadow is a "drab reflections of lifeless objects present in the physical world." (p. 112 - Shadows)
p. 114 - Astral Visibility :
Shadows of physical objects in the astral plane may be drab and insubstantial, but they are still opaque and can prevent targeting. Items that are transparent or mirrored in the real world (like a car window) simply impair visibility as astral shadows.
So in this scenario, the adept could not see through the window since glass, as any other astral shadow, is opaque to Astral Perception.
The only exception to shadows impairing astral visibility is clothing (Street Magic p.112 - Auras) :
clothes and other non-living objects are often outshone by the brightness of the wearer’s aura
Contrary to cyberware which, by its intrusive nature, leaves shadowy gaps in auras, which are easily perceived by an Assenssing check with 2+ successes, while bioware, being living tissue, needs at least 4 successes (SR4A p.191 - Assensing Table)
EDIT : the sentence "simply impair visibility" being vague at best, let me extrapolate from other sources :
SR4A p.191 - Astral Perception :
Astral perception is a psychic sense that is not linked to the character’s physical sight.
While there may be some material "simply impairing" (that is : a material which you can astrally "see" through, but with some malus) a psychic sense, I can think of no reason why simple glass would be such a material. Glass' ability to let light go through itself doesn't justify an ability to let emotions do the same.
As quoted in Trajan's answer, p.114 of Street Magic states "Shadows of physical objects in the astral plane may be drab and insubstantial, but they are still opaque and can prevent targeting. Items that are transparent or mirrored in the real world (like a car window) simply impair visibility as astral shadows."
If transparent items "simply impair visibility" instead of being opaque, then they are not opaque. They are difficult, but not impossible, to see through and would impose the normal penalties for poor visibility.
(This would be more explicitly clear if the first sentence read "opaque objects" rather than "physical objects", since a car window is still a "physical object", but it is still structurally clear that the second sentence is drawing a contrast between transparent or mirrored items and the objects described in the first sentence.)
