Answer:
Cold cathode lamps are a type of gas discharge lamp that operate at a low pressure and do not require a heated filament to emit light. Among the options provided, the correct answer is D: Neon lamp.
Here is a detailed explanation for each option:
- Sodium vapour lamp: Sodium vapour lamps are a type of high-intensity discharge lamp that use an electric arc through sodium vapor to produce light. They do not fall under the category of cold cathode lamps.
- High pressure mercury vapour lamp: High pressure mercury vapour lamps also fall under the category of high-intensity discharge lamps. They use an electric arc through mercury vapor to produce light and require a heated filament. They are not cold cathode lamps.
- Low pressure mercury vapour lamp: Low pressure mercury vapour lamps are a type of fluorescent lamp that operate at a low pressure and require a heated filament to emit electrons. They are not cold cathode lamps.
- Neon lamp: Neon lamps are a type of cold cathode lamp that use neon gas to produce light. They operate at a low pressure and do not require a heated filament. Neon lamps are commonly used for decorative and indicator purposes.
In conclusion, among the given options, the only cold cathode lamp is the Neon lamp.

Neon lamp is a cold cathode lamp. Neon lamps are a type of gas discharge lamp that operates using low-pressure neon gas. The lamp contains a sealed glass tube filled with neon gas and two electrodes at each end. When a high voltage is applied across the electrodes, the neon gas inside the tube becomes ionized, and the lamp produces a glowing light.

Neon lamps are called cold cathode lamps because they do not rely on a heated filament like incandescent lamps or hot cathodes like fluorescent lamps. Instead, they use a cold cathode, which is an electrode that does not require heating to emit electrons. The cold cathode in a neon lamp is typically made of a material with a high electron emission coefficient, such as tungsten or other alkaline earth metals.

The operation of a neon lamp involves the following steps:

1. Ionization: When a high voltage is applied across the electrodes, it creates a strong electric field inside the lamp. This electric field ionizes the neon gas, causing some of the gas atoms to lose electrons and form positive ions.

2. Electron emission: The high electric field near the cold cathode causes the emission of electrons from the cathode surface. These electrons are accelerated towards the anode (the other electrode), while the positive ions move towards the cathode.

3. Electron collision: As the electrons move towards the anode, they collide with the positive ions and other gas atoms. These collisions excite the gas atoms, causing them to emit photons of light.

4. Light emission: The photons emitted by the excited gas atoms give the neon lamp its characteristic glowing light. The color of the light depends on the specific gas used and any additional coatings on the inside of the lamp.

Neon lamps have several applications, including signage, indicator lights, and decorative lighting. They are known for their long lifespan, high visibility, and low power consumption. However, they require a high voltage to operate, typically ranging from a few hundred volts to several thousand volts.

In summary, a neon lamp is a cold cathode lamp that uses low-pressure neon gas to produce a glowing light. It operates by ionizing the gas and emitting electrons from a cold cathode, resulting in the emission of photons and the characteristic glow.