This is an excellent question. A silvered mirror is a piece of glass, coated on one side with a very reflective metal. The silvered side with have an extremely low emissivity, but the other side is essentially glass, so its emissivity will be the emissivity for the glass, which should be high.

Sometimes people use silvered mirrors as infrared mirrors, to detect reflected radiation (usually when no direct line of sight is available). It is important to position the mirror the right way around.

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