When white light passes through a glass prism and forms a spectrum, which colour ray in the emergent beam is deviated the least?

Introduction / Context:
When white light passes through a glass prism, it is dispersed into its constituent colours, forming a spectrum from violet to red. Each colour corresponds to a different wavelength and is refracted by a slightly different amount. This question tests your understanding of which colour experiences the smallest deviation as it emerges from the prism, which relates directly to how refractive index varies with wavelength.

Given Data / Assumptions:
- Incident light is white light containing all visible wavelengths.
- The medium is a glass prism, which causes refraction and dispersion.
- Options list different colours of light: violet, red, green and blue.

Concept / Approach:
The refractive index of glass is generally higher for shorter wavelengths (violet) and lower for longer wavelengths (red). A higher refractive index means greater bending towards the normal and therefore greater deviation through the prism. Violet light, with the shortest wavelength, is deviated the most, while red light, with the longest wavelength in the visible spectrum, is deviated the least. Intermediate colours like blue and green experience deviations between those extremes.

Step-by-Step Solution:
Step 1: Recall that dispersion causes different colours in white light to refract by different amounts. Step 2: Note that shorter wavelength colours (like violet) have larger refractive indices in glass and are bent more. Step 3: Longer wavelength colours (like red) have smaller refractive indices and are bent less. Step 4: Therefore, as white light emerges from the prism, red light appears at the top of the spectrum and has experienced the smallest deviation. Step 5: Conclude that the red ray is deviated least.

Verification / Alternative check:
Standard diagrams of dispersion through a glass prism show a spectrum with violet at the bottom and red at the top (for the usual orientation), indicating that violet deviates most and red deviates least. Textbooks often explicitly state that in a prism "red light suffers the least deviation" and "violet light suffers the greatest deviation". This visual and textual evidence confirms the answer.

Why Other Options Are Wrong:
The violet ray: Violet has the shortest wavelength in visible light and hence the largest refractive index in glass, so it is deviated the most, not the least.
The green ray: Green lies between blue and yellow in the spectrum and experiences intermediate deviation, not the minimum.
The blue ray: Blue has a shorter wavelength than green and red, and is deviated more than green and much more than red.

Common Pitfalls:
Some learners incorrectly assume that the colour appearing at one end of the spectrum must be most or least deviated without remembering which colour has shorter or longer wavelength. Others confuse refractive index behaviour with that of diffraction. To avoid mistakes, remember the key pattern: in a prism, red has the longest wavelength and least deviation, while violet has the shortest wavelength and greatest deviation. The famous sequence VIBGYOR helps you recall the order of colours.

Final Answer:
In the emergent beam, the ray deviated the least is the red ray.