A narrow beam of white light can be split up into different Colours by a glass prism. The correct explanation is that

Analysis

The question asks for the physical reason why a glass prism splits white light into its constituent colors (a phenomenon known as dispersion).

Concept Explanation

White light is composed of a spectrum of colors (Red, Orange, Yellow, Green, Blue, Indigo, Violet). When white light enters a glass prism, it undergoes refraction (bending).

The key principle here is the Refractive Index ($n$). The refractive index of a material is defined as the ratio of the speed of light in a vacuum ($c$) to the speed of light in that medium ($v$):

In a glass prism, the speed of light depends on its frequency (or wavelength). Different colors have different frequencies; therefore, they travel at different speeds within the glass. Because they travel at different speeds, each color is refracted (bent) by a different angle. Violet light travels the slowest and bends the most, while red light travels the fastest and bends the least. This separation of speeds is what causes the white light to spread out into a spectrum.

Step-by-Step Solution

1. Analyze the options: • A: While true that light is an electromagnetic wave, this property alone does not explain why it splits into colors when passing through a prism. • B: The prism does not "contain" the colors; it merely acts as a medium that separates the colors already present in the white light. • C: Because the refractive index of glass varies with the wavelength of light, each color travels at a different speed ($v = c/n$). This difference in speed causes the different angles of refraction, leading to dispersion. • D: Total internal reflection occurs when light travels from a denser medium to a less dense medium at an angle greater than the critical angle. This is not the mechanism behind the splitting of white light into a spectrum.

2. Conclusion: The splitting of white light (dispersion) is fundamentally caused by the variation in the speed of different colors of light as they pass through the glass medium.

Final Answer

The correct explanation is that different colors of white light travel with different speeds in glass, leading to different degrees of refraction.

Correct Option: C

Exam Tip

Remember the acronym ROYGBIV (Red, Orange, Yellow, Green, Blue, Indigo, Violet). Red has the longest wavelength and travels the fastest in glass (bends the least), while Violet has the shortest wavelength and travels the slowest in glass (bends the most). This relationship is a favorite topic in JAMB Physics!