Understanding Monochromatic Light and Different Lamps

Monochromatic light is light of a single color or, more precisely, light having a very narrow range of wavelengths. Truly monochromatic light consists of only one wavelength, but in practice, sources are considered nearly monochromatic if they emit light primarily at one or a few very closely spaced wavelengths.

Analyzing the Light Spectrum of Different Lamp Types

Let's examine the light characteristics of the lamps mentioned in the options:

• Tube light (Fluorescent lamp): These lamps produce light by exciting a gas (usually mercury vapor) which then emits ultraviolet light. This UV light hits a phosphor coating on the inside of the tube, causing it to fluoresce and emit visible light. The resulting light is a mixture of discrete spectral lines from the gas and a broad spectrum from the phosphors. It produces a wide range of wavelengths, not monochromatic light.
• Sodium vapour lamp: These lamps contain sodium metal which, when heated and excited by an electric current, emits light. Low-pressure sodium vapour lamps primarily emit light at two very closely spaced wavelengths in the yellow part of the spectrum (the sodium D lines at λ = 589.0 nm and λ = 589.6 nm). This light is perceived as a very pure yellow and is considered nearly monochromatic because the two dominant wavelengths are extremely close. High-pressure sodium lamps produce a broader spectrum, but low-pressure ones are famous for their nearly monochromatic output.
• GLS lamp (General Lighting Service lamp / Incandescent lamp): These lamps produce light by heating a filament to a high temperature. The filament emits light across a continuous spectrum of wavelengths, from infrared through visible red, orange, yellow, green, blue, and violet. It produces a broad, continuous spectrum, not monochromatic light.
• Mercury vapour lamp: These lamps also work by passing an electric current through mercury vapor. They emit light at specific, discrete wavelengths (spectral lines), predominantly in the blue, green, and ultraviolet regions. While it emits specific wavelengths (a line spectrum), it includes several distinct lines across the visible spectrum, making it polychromatic (multiple colors/wavelengths) rather than monochromatic.

Identifying the Nearly Monochromatic Light Source

Comparing the light output of these lamps, the sodium vapour lamp, particularly the low-pressure type, is known for emitting light that is highly concentrated in a very narrow band of wavelengths around the yellow region. This characteristic makes it the lamp that produces nearly monochromatic light among the given options.

Therefore, the sodium vapour lamp is the correct answer as it provides nearly monochromatic light.

Revision Table: Lamp Types and Light

Additional Information on Light Sources and Spectra

Different light sources produce light in different ways, resulting in different types of spectra. Understanding these spectra is key to understanding the properties of light from various sources:

• Continuous Spectrum: Emitted by incandescent solids or liquids. All wavelengths within a certain range are present, like the light from a regular light bulb or the sun.
• Line Spectrum: Emitted by excited gases. Light is produced only at specific, discrete wavelengths, resulting in sharp lines when viewed through a spectroscope. Examples include neon signs or discharge lamps like mercury or sodium vapor lamps.
• Monochromatic Light Sources: While truly monochromatic sources are rare outside of lasers, some discharge lamps like low-pressure sodium lamps approximate this by emitting light concentrated in a very narrow band. Lasers, on the other hand, are highly monochromatic, emitting light with a very precise wavelength.

The term "monochromatic" comes from Greek words "mono" meaning single and "chroma" meaning color. Thus, it literally means "single color".