In the visible region of the electromagnetic spectrum, which colour of light has the longest wavelength?

Introduction / Context:
This question tests basic knowledge of the visible spectrum of light, a topic commonly illustrated by the rainbow or a prism experiment. Visible light consists of a range of colours from violet to red, each associated with a different wavelength and frequency. Knowing which colour corresponds to the longest wavelength is important in optics, communication and general science questions. This fact also connects with the well known ordering of colours in a rainbow.

Given Data / Assumptions:

• We are considering only the visible region of the electromagnetic spectrum.
• The common rainbow order of colours is violet, indigo, blue, green, yellow, orange and red.
• The options given are violet, yellow, red and white.
• We must identify which single colour has the longest wavelength.

Concept / Approach:
The visible spectrum can be arranged in order of increasing wavelength or increasing frequency. When arranged in order of increasing wavelength, violet has the shortest wavelength and red has the longest wavelength. Frequency is inversely related to wavelength, so violet has the highest frequency and red the lowest frequency among visible colours. White light is not a single wavelength but a mixture of many visible wavelengths. Therefore, among the individual colours listed, red light clearly corresponds to the longest wavelength in the visible range.

Step-by-Step Solution:
Step 1: Recall the standard order of colours in a rainbow using the mnemonic such as VIBGYOR: Violet, Indigo, Blue, Green, Yellow, Orange, Red. Step 2: Remember that in this order, violet is at the short wavelength, high frequency end, and red is at the long wavelength, low frequency end. Step 3: Compare the options. Violet is at the short wavelength end, so it cannot have the longest wavelength. Step 4: Yellow lies between green and orange and therefore has a wavelength intermediate between those colours, less than red. Step 5: White light is a mixture of all visible wavelengths and is not a single wavelength colour. Step 6: Conclude that red light has the longest wavelength among the visible colours listed.

Verification / Alternative check:
As a cross check, think about dispersion through a glass prism. When white light passes through a prism, violet deviates the most and red deviates the least. This behaviour occurs because violet light has a shorter wavelength and therefore experiences greater change in speed and bending in the glass compared to red. The smaller deviation of red corresponds to its longer wavelength. Such prism experiments are standard in school physics and confirm the wavelength ordering of visible colours.

Why Other Options Are Wrong:
Violet light has the shortest visible wavelength and therefore cannot be the longest. Yellow light has a wavelength in the middle of the visible spectrum and is shorter than red. White light is not a single colour but a mixture of many wavelengths, so talking about a single longest wavelength for white is not meaningful.

Common Pitfalls:
Some learners get confused between wavelength and frequency and mistakenly think that the colour that appears most intense or brightest must have the longest wavelength. Others may not clearly remember the order of colours in the spectrum. To avoid such errors, memorise the simple rule that in visible light violet has the shortest wavelength and red has the longest, and remember the standard VIBGYOR ordering from violet to red.

Final Answer:
In the visible spectrum, the colour with the longest wavelength is Red.