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Light passing through a PRISM is dispersed because different colours of light have
Adifferent speeds in vacuum
Bdifferent refractive indices in the glass (dispersion)
Cdifferent frequencies in the glass
Ddifferent intensities of incident light
Answer & Solution
Correct answer: B. different refractive indices in the glass (dispersion)
1. NCERT §9.7 (Refraction through a Prism + Dispersion): the refractive index of any medium depends on the WAVELENGTH of light.
2. Typically $n$ DECREASES with wavelength (normal dispersion): violet light has higher $n$ than red light in glass.
3. Snell's law then bends different colours by different amounts: violet bends MORE than red. This is DISPERSION.
4. After two refractions through a prism (entry + exit), the colours emerge in a fan of different deviation angles — the visible spectrum.
5. Newton's classic experiment confirmed white light is a mixture; the prism doesn't 'colour' the light, it just separates the components.
6. Option A is wrong — in vacuum, all colours have the same speed $c$. Option C confuses frequency (which is unchanged across media) with refractive index. Option D ignores the physics.
_Source: NCERT Class 12 Physics Part 2, Ch 9, §9.7 (Refraction through a Prism — dispersion by a prism), p. 22–24._
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