When a white light beam is made to fall on a hollow prism filled with water, it breaks into seven constituent colours. Which of the following colours suffers the maximum angle of deviation?

Correct — B, Violet. A prism separates white light because the refractive index of the medium is not the same for every colour — it is largest for the shortest wavelength. Violet has the shortest wavelength of the seven, so it travels slowest inside the water, is refracted most strongly at both faces, and emerges with the largest angle of deviation. Red is deviated least. That is why the spectrum from a prism always runs red at the top and violet at the bottom. A hollow prism filled with water behaves as a water prism and disperses for exactly the same reason, though less strongly than glass.

• (a)Red — Red has the longest wavelength and the smallest refractive index in water, so it is the least deviated colour — the opposite end of the same spectrum.
• (c)Green — Green sits in the middle of the visible band, so its deviation is intermediate. No colour between red and violet can be the maximum.
• (d)All suffer by equal amount of deviation — If all colours were deviated equally there would be no dispersion at all and the beam would emerge white. The seven colours described in the stem are themselves proof that the deviations differ.

Dispersion is the splitting of white light into its constituent colours, and it happens because refractive index depends on wavelength. Deviation through a prism increases with refractive index, and refractive index increases as wavelength decreases, so the order of deviation is the reverse of the order of wavelength: violet most, red least. Newton established this with a glass prism and then recombined the colours with a second prism to show that white light is a mixture.

The chain of reasoning has three links and candidates lose it at the second. Shortest wavelength gives the highest refractive index; the highest refractive index gives the slowest speed in the medium; the slowest speed gives the greatest bending. A candidate who reasons that violet is 'more energetic so it goes straight through' inverts the middle link and picks red. The hollow-prism detail is a further test — the prism walls are thin and the dispersing medium is the water inside, so the physics is unchanged.

• Refractive index increases as wavelength decreases, so it is greatest for violet.
• Deviation through a prism is greatest for violet and least for red.
• The spectrum from a prism runs red at the top and violet at the bottom.
• A rainbow forms by refraction, dispersion and internal reflection inside raindrops.
• Recombining the spectrum with a second inverted prism restores white light, as Newton demonstrated.

• Reversing the wavelength-to-deviation link and answering red.
• Assuming that filling the prism with water rather than glass changes which colour bends most.
• Choosing the equal-deviation option, which would mean no spectrum at all.

A standard dispersion item; the NDA versions ask the same fact with the reason attached, so the candidate must match colour and refractive index in one step.

• practice — not a real PYQ

  In the spectrum produced by a glass prism, the colour deviated the least is

  • (a)violet
  • (b)green
  • (c)yellow
  • (d)red

  Answer(d) red — the longest wavelength has the smallest refractive index and so the smallest deviation.

• practice — not a real PYQ

  White light passing through a prism splits into colours because

  • (a)the prism adds colour to the light
  • (b)the refractive index of the prism differs for different wavelengths
  • (c)the light is reflected inside the prism
  • (d)the prism absorbs some wavelengths

  Answer(b) the refractive index of the prism differs for different wavelengths — that is what dispersion means.