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A window
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Suggesting that the object can emit or let through any wavelengths within the visible spectrum. Windows are made of transparent material by nature, allowing it to appear in any color (light emitted by objects behind it).
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Windows are so commonplace that they are virtually unavoidable within our daily lives (buildings, cars, etc.), and can thus be seen every day.
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Early documentation of windows date back to several centuries BC.
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By sitting down, you are more likely to be in a place surrounded by windows (e.g. sitting down on a seat inside a building) or the object becomes (more) apparent, for instance by sitting down on a train passenger seat.