AIIMS 2003 Physics Light Emission Assertion Reason Question

To analyze the provided assertion and reason, let's break down each component:

Assertion:

A tube light emits white light.
This assertion is true. Tube lights (or fluorescent lamps) indeed emit light that appears white to the human eye. This white light is produced due to a combination of phosphors inside the tube that fluoresce when excited by ultraviolet light generated by the ionized gas within the tube.

Reason:

Emission of light in a tube takes place at a very high temperature.
This reason is false. The emission of light in a tube light does not occur at very high temperatures like in incandescent bulbs, where the filament must be heated to thousands of degrees Celsius to produce light. Instead, tube lights operate at relatively low temperatures. The light is produced through the excitation of gas (usually a mixture of argon and mercury vapor) which, when ionized, emits ultraviolet light. This ultraviolet light then excites the phosphor coating inside the tube, resulting in visible light. Therefore, the temperature involved in this process is not high in comparison to incandescent bulbs.

Conclusion on Options:

• A) Both assertion and reason are true, and the reason is the correct explanation of the assertion.
  Incorrect. The assertion is true, but the reason is false.

• B) Both assertion and reason are true, and the reason is not the correct explanation of the assertion.
  Incorrect. The assertion is true, but the reason is false.

• C) Assertion is true but reason is false.
  Correct. The assertion is true, and the reason is indeed false.

• D) Both assertion and reason are false statements.
  Incorrect. The assertion is true, so this option cannot be correct.

Correct Answer:

Thus, the correct answer is C) if assertion is true but reason is false.

Key Concepts:

1. Fluorescence: A process where a material absorbs light or other electromagnetic radiation and then re-emits it. Fluorescent materials convert UV light to visible light.
2. Incandescence: A process where light is produced due to the high temperature of a heated filament (like in traditional bulbs). The temperature can reach around 2500-3000 K.
3. Operating Principle of Tube Lights: A fluorescent tube light operates on the principle of gas discharge and fluorescence, not on high temperature.

Summary:

In conclusion, the assertion is indeed correct because tube lights emit white light, but the reason provided is incorrect since the emission does not occur at high temperatures. Therefore, option C is the correct choice.