Identifying Lamps with Inner Discharge and Outer Evacuated Tubes

The question asks to identify the type of lamp that consists of an inner discharge tube and an outer evacuated tube. Let's examine the structure of the lamp types mentioned in the options:

Understanding Lamp Structures

Different types of lamps use various designs to produce light efficiently and safely. The structure involving an inner discharge tube within an outer envelope serves specific purposes, such as maintaining optimal operating temperature, providing thermal insulation, or protecting the inner components from the external environment.

• Fluorescent Lamp: A standard fluorescent lamp is typically a single glass tube internally coated with phosphor. It contains a low-pressure mixture of mercury vapour and an inert gas like argon. While it has electrodes at each end and operates via a discharge, it doesn't typically have a separate inner discharge tube within an evacuated outer tube in the same way as some other high-intensity discharge lamps.
• Sodium Lamp: Sodium lamps come in two main types: low-pressure sodium (LPS) and high-pressure sodium (HPS).
  • LPS lamps have a U-shaped inner discharge tube made of special glass resistant to sodium, contained within a vacuum flask to provide thermal insulation.
  • HPS lamps have a narrow, high-temperature resistant ceramic arc tube (the discharge tube) contained within an outer glass bulb, which is often evacuated or filled with an inert gas. This structure fits the description of an inner discharge tube within an outer evacuated tube.
• High Pressure Mercury Vapour Lamp: These lamps typically consist of a quartz arc tube (the inner discharge tube) where the mercury vapour discharge occurs. This arc tube is enclosed within a larger, hard glass outer bulb. The space between the arc tube and the outer bulb is usually evacuated or filled with an inert gas like argon. This outer envelope provides thermal insulation to keep the arc tube at a high temperature for efficient operation and also filters out UV radiation produced by the discharge. This structure directly matches the description given in the question: an inner discharge tube (quartz arc tube) and an outer evacuated tube (the outer glass bulb with vacuum).

Analysis Based on Structure

Based on the typical structures of these lamps:

• Fluorescent lamps do not fit the description of having a separate inner discharge tube within an outer evacuated tube.
• Both types of Sodium lamps have an inner tube within an outer envelope, often evacuated, so they could potentially fit.
• High Pressure Mercury Vapour lamps explicitly feature an inner quartz arc tube within an outer glass bulb that is commonly evacuated, fitting the description well.

Considering the options provided and the common structures of these lamps, the High Pressure Mercury Vapour lamp is a classic example described by having an inner discharge tube and an outer evacuated tube.

The question describes a lamp with an inner discharge tube and an outer evacuated tube. While High Pressure Sodium lamps also fit this general description, the structure is particularly characteristic and widely associated with High Pressure Mercury Vapour lamps where the outer bulb being evacuated is crucial for thermal management and safety (UV absorption). Therefore, the High Pressure Mercury Vapour lamp is the type that fits the description.

Revision Table: Lamp Components

Additional Information on High Pressure Mercury Vapour Lamps

High pressure mercury vapour lamps are a type of high-intensity discharge (HID) lamp. They produce light by passing an electric arc through vaporized mercury at high pressure. The initial discharge often happens through a small amount of argon gas mixed with the mercury. As the lamp warms up, the mercury vaporizes, and the pressure increases, leading to a brighter and more efficient light output. The outer bulb often contains a phosphor coating on the inside surface to improve the colour rendering index (CRI) by converting some of the ultraviolet radiation into visible light. These lamps require a ballast to regulate the current and voltage during operation.