Here is the transparency shader. This is also called a "thin glass" shader. Thin glass is unable to significantly bend light rays because they enter and exit the glass almost in the same spot. The exit bend cancels the enter bend before the ray is much displaced. So we can cheat and not use refraction at all for thin glass. Some renderers also call this material "architectural" glass - again they just mean that it is safe to ignore bending refractions and you can use a fast shader.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)