Turning on.
See the gas flicker within the florescent glass tubes.

Picture:
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The effect:
What it does to a white T-shirt when turned on

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Summary:
When the black light gets turned, the gas reacts to the electricity inside. This gives it the florescent light. The glass tube it's self is coated a purple/black.

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Summary:
When the black light is on, it makes all bright colors including white brightly glow coolly.