A domestic fluorescent tube light produces white light. Which option best explains the physical reason for the white fluorescent light emitted by such a tube?

Introduction / Context:
This question relates to the working principle of fluorescent tube lights, which are widely used in homes and offices. Unlike incandescent bulbs that produce light by heating a filament, fluorescent lamps rely on gas discharge and fluorescence. Understanding why the light appears white requires knowing how ultraviolet radiation and phosphor coatings interact inside the tube.

Given Data / Assumptions:

• The device considered is a domestic fluorescent tube light.
• The tube contains a low pressure gas (often mercury vapour) and has a white coating on the inner surface of the glass.
• Electric current is passed through the tube using a starter and choke.
• We are asked why the tube finally emits white fluorescent light.

Concept / Approach:
Inside a fluorescent tube, the electric discharge through low pressure mercury vapour produces mainly ultraviolet (UV) radiation, not visible light. The inner surface of the glass tube is coated with a phosphor material, which absorbs the ultraviolet photons and re emits energy as visible light of various wavelengths. The mixture of these visible wavelengths appears as white light to our eyes. The key step is the conversion of invisible ultraviolet radiation into visible light by the phosphor coating. Simply having heavy current or a vacuum does not explain the white fluorescence.

Step-by-Step Solution:
Step 1: When the tube is switched on, a high voltage starts an electric discharge through low pressure mercury vapour inside the tube. Step 2: Collisions between electrons and mercury atoms excite the atoms, which then emit photons primarily in the ultraviolet range. Step 3: Ultraviolet light produced inside the tube cannot be seen directly by the human eye and would be harmful if emitted strongly. Step 4: The inner surface of the tube is coated with a white phosphor material that is sensitive to ultraviolet radiation. Step 5: When ultraviolet photons fall on the phosphor coating, the coating absorbs this energy and re emits it as visible light of many wavelengths. Step 6: The combination of these visible wavelengths forms a white appearing light, which is what we see coming from the fluorescent tube.

Verification / Alternative check:
If the inner coating is damaged or missing, the tube would not produce proper white light. In such a case, more ultraviolet radiation would escape, and the brightness would decrease or the colour would change. Also, compact fluorescent lamps (CFLs) use the same principle: a gas discharge produces UV, which is converted to visible light by a phosphor coating. These practical observations support the explanation that the key step involves ultraviolet radiation falling on a white phosphor coating, not just collisions or vacuum alone.

Why Other Options Are Wrong:
Collision between molecules of filled gas under electric current is part of the process that produces ultraviolet radiation, but by itself it does not explain the white fluorescent light observed; the role of the phosphor coating is missing. Heavy current is not the main reason for fluorescence; even moderate currents can produce light if the discharge and coating are correct. Vacuum inside the tube is incorrect; fluorescent tubes contain low pressure gas, not a complete vacuum, and the emission mechanism relies on gas discharge.

Common Pitfalls:
Students often give overly generic answers such as electric discharge or heavy current without mentioning the critical role of ultraviolet radiation and the phosphor coating. Another confusion is to assume that the gas itself directly emits white light. Remember that in fluorescent tubes, the gas primarily produces UV, and the coating converts it to visible light. Recognising this two step process will help you answer similar questions correctly.

Final Answer:
A fluorescent tube gives white light mainly because ultra violet rays produced inside the tube fall on the white inner coating (phosphor), which then emits visible white light.