MadSci Network: Physics

Re: How do fluorescent neon tube lights produce a striped zebra effect?

Date: Sat Oct 30 17:14:35 2010
Posted By: Adrian E. Popa, Retired Laboratory Director
Area of science: Physics
ID: 1288271096.Ph

Question: How do fluorescent neon tube lights produce a striped zebra effect?

Area: Physics Message ID Number: 1288271096.Ph

Greetings David:

Reference 1. Phosphors Phosphors

Reference 2 Fluorescent light bulbs

http://en.wikipedia.or g/wiki/Fluorescent_lamp http://en.wikipedia.or g/wiki/Fluorescent_lamp

Reference 3 How fluorescent light bulbs work.

Visible white light is efficiently produced from a fluorescent light bulb by exciting a
coating of phosphors inside a glass tube with invisible ultraviolet (UV) radiation.
See Reference 1 about how phosphors can produce light. The bulb is usually filled with
an inert gas such as argon and a small amount of mercury is placed within the tube. See
References 2 and 3 for pictures of the electrical circuit. The electrical voltage ionizes
the mercury atoms to produce UV light. Ionization changes some of the mercury in the
tube from a liquid to a gas. However, these atoms do not emit UV light continually--instead,
their energy levels rise and fall, like thousands of miniature lightning bolts, giving off
UV light only when they become energized. If the voltage is large enough the entire volume
of gas in the tube will light.

If the voltage that excites the fluorescent tube is low, which is often the case in
electrically powered trains because they consume so much power, there is not enough voltage
to excite the whole tube and only portions of the tube may light up, often in bands (stripes)
of light. Because of thermal heating and electrical field effects the hot gasses within the
tube also move in a stirring movement. This is similar to lighting bolts which move from
place to place as the storm moves. Therefore the gas that is forming the bands of light
in the tube will also move, generating moving patterns of light. As the train starts it requires
maximum electrical power and the voltage will drop to a minimum. As the train moves at
speed the voltage will rise, and as the train slows down they often generate greater voltage
by electrical braking using the trains electrical motors as generators. Thus as the train
moves the changes in voltage will change and the number of light patterns produced. Also,
the voltage changes will also change the movement of the patterns.

Thank you for an interesting question.

Best regards, Your Mad Scientist
Adrian Popa

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