Understanding Monochromatic Visible Light

The question asks about the characteristics of monochromatic visible light. The term "monochromatic" itself gives us a big clue. Let's break down the word:

• 'Mono' means single or one.
• 'Chromatic' relates to colour.

So, literally, monochromatic means single colour.

In physics, the colour of light is determined by its wavelength. Different wavelengths of visible light correspond to different colours in the spectrum (like red, orange, yellow, green, blue, indigo, violet). For light to have a single colour, it must consist of light waves that all have the same, or very nearly the same, wavelength.

Therefore, monochromatic visible light is light that consists of only one specific wavelength (or a very narrow range of wavelengths), which results in it having a single, pure colour.

Analyzing the Options

Let's look at the given options based on our understanding:

• Option 1: "Light of a single wavelength with all the colours of the spectrum of white light." This is incorrect. If light has a single wavelength, it can only have one specific colour, not all colours of the spectrum. White light is a combination of many wavelengths and thus many colours.
• Option 2: "Light of many wavelengths with a single colour." This is generally incorrect. Different wavelengths correspond to different colours. Light with many wavelengths is typically polychromatic (multiple colours), like white light or mixed colours. While some combinations might appear as a certain colour (e.g., mixing red and green light appears yellow), true single colour, monochromatic light is defined by a single wavelength.
• Option 3: "Light of many wavelengths with all the colours of the spectrum of white light." This describes polychromatic light, specifically white light. It has many wavelengths and contains all the visible colours, so it is not monochromatic.
• Option 4: "Light of a single colour and single wavelength." This aligns perfectly with the definition of monochromatic light. A single wavelength corresponds to a single, pure colour in the visible spectrum.

Conclusion on Monochromatic Light

Based on the analysis, monochromatic visible light is characterized by having a single colour, which is a direct result of it having a single wavelength (or a very narrow band of wavelengths). Lasers are common examples of devices that produce highly monochromatic light.

Revision Table: Key Properties of Monochromatic Light

Additional Information: Wavelength and Colour

Visible light is a form of electromagnetic radiation. The human eye perceives different wavelengths within the visible spectrum as different colours. The approximate ranges are:

• Violet: $\lambda \approx 380 - 450 \, \text{nm}$
• Blue: $\lambda \approx 450 - 495 \, \text{nm}$
• Green: $\lambda \approx 495 - 570 \, \text{nm}$
• Yellow: $\lambda \approx 570 - 590 \, \text{nm}$
• Orange: $\lambda \approx 590 - 620 \, \text{nm}$
• Red: $\lambda \approx 620 - 750 \, \text{nm}$

Where $\lambda$ represents the wavelength in nanometers (nm). Monochromatic light, ideally, would have just one specific value of $\lambda$ within one of these ranges, corresponding to a pure colour.