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When a light beam falls on a triangular glass prism, a band of colours

June 1, 2025 by rawan239

When a light beam falls on a triangular glass prism, a band of colours is obtained. Which one of the following statements is correct in this regard?

Red light bends the most, as the refractive index of glass for red light is greatest.

Red light bends the most, as the refractive index of glass for red light is lowest.

Violet light bends the most, as the refractive index of glass for violet light is greatest.

Violet light bends the most, as the refractive index of glass for violet light is lowest.

This question was previously asked in

UPSC NDA-2 – 2021

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The correct answer is that Violet light bends the most because the refractive index of glass for violet light is greatest.

When white light passes through a prism, it splits into its constituent colours due to dispersion. The amount of bending (deviation) of light by a prism depends on the refractive index of the prism material for that specific colour. According to Cauchy’s formula and experimental observations, the refractive index of a material decreases as the wavelength of light increases. Violet light has the shortest wavelength among the visible colours, while red light has the longest wavelength. Therefore, the refractive index of glass is highest for violet light and lowest for red light. A higher refractive index leads to a greater deviation (bending).

The deviation of light by a prism is given approximately by $\delta = (\mu – 1)A$, where $\mu$ is the refractive index of the prism material and A is the angle of the prism. Since $\mu_{\text{violet}} > \mu_{\text{red}}$, it follows that $\delta_{\text{violet}} > \delta_{\text{red}}$. This causes violet light to be deviated the most and red light the least, resulting in the separation of colours observed as a spectrum.

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