Understanding Light Deviation by a Glass Prism

When white light passes through a glass prism, it splits into its constituent colours. This phenomenon is known as dispersion. The splitting happens because the refractive index of the glass material is different for different wavelengths (colours) of light.

The deviation angle ($\delta$) of light passing through a prism depends on the refractive index ($n$) of the prism material for that specific wavelength of light. Generally, the deviation increases with increasing refractive index. The relationship between deviation, angle of prism ($A$), and refractive index ($n$) for small angles is approximately $\delta = (n-1)A$.

The colours of white light are arranged in a spectrum often remembered by the acronym VIBGYOR:

• Violet
• Indigo
• Blue
• Green
• Yellow
• Orange
• Red

These colours have different wavelengths. Red light has the longest wavelength, while violet light has the shortest wavelength.

For most transparent materials like glass, the refractive index is higher for shorter wavelengths and lower for longer wavelengths. This means:

• The refractive index for violet light ($n_{violet}$) is the highest.
• The refractive index for red light ($n_{red}$) is the lowest.

Since deviation is proportional to $(n-1)$, a higher refractive index leads to a greater deviation, and a lower refractive index leads to a smaller deviation.

• Violet light, having the highest refractive index, is deviated the most.
• Red light, having the lowest refractive index, is deviated the least.

Therefore, among the given options, the colour that is least deviated by the glass prism is red colour.