The coloured light that refracts most while passing through a prism is
2023
The coloured light that refracts most while passing through a prism is
Answer: B. violet — Concept — Refraction bends a ray because the refractive index n of the medium differs from that of the surrounding air, and n is not a single number for a…
- A.
yellow
- B.
violet
- C.
blue
- D.
More than one of the above
- E.
None of the above
Attempted by 2 students.
Show answer & explanation
Correct answer: B
Concept — Refraction bends a ray because the refractive index n of the medium differs from that of the surrounding air, and n is not a single number for a material: it depends on the wavelength of the light passing through it. In every ordinary transparent medium n grows steadily as the wavelength shrinks — normal dispersion, captured by Cauchy's relation n = a + b/λ2. Snell's law makes the bending at each face grow with n, and for a thin prism of refracting angle A the total deviation is δ = (n − 1)A, so the deviation rises steadily with the refractive index.
Application
Glass is a dispersive medium, so every colour entering the prism meets its own refractive index, fixed by that colour's wavelength.
Rank the colours named in the question by wavelength: yellow lies near 580 nm, blue near 470 nm and violet near 410 nm.
Because n grows as the wavelength falls, the refractive indices of a typical borosilicate crown glass run the opposite way: close to 1.517 for yellow, close to 1.522 for blue and close to 1.530 for violet.
Substitute these in δ = (n − 1)A: the deviation is greatest for the greatest refractive index, so violet is bent the most on passing through a prism.
Comparison
Colour | Approx. wavelength | n (borosilicate crown glass) | Deviation δ = (n − 1)A |
|---|---|---|---|
Yellow | ≈ 580 nm | ≈ 1.517 | smallest of the three |
Blue | ≈ 470 nm | ≈ 1.522 | intermediate |
Violet | ≈ 410 nm | ≈ 1.530 | greatest of the three |
Cross-check
A prism spreads white light into the band red, orange, yellow, green, blue, indigo, violet; violet emerges at the far end, turned furthest from the original direction, while red is turned least. That familiar spectrum is the everyday form of the same n-versus-wavelength rule.
Two of the named colours could share the greatest bending only if they shared one refractive index; ordinary glass shows normal dispersion across the visible range, its refractive index falling steadily as the wavelength rises, so colours of different wavelength take different values.
All three colours named belong to the visible spectrum, and among them the greatest refractive index goes with the shortest wavelength, violet, so no colour outside the list is required.