A student passes white light through a glass prism and observes a spectrum on a screen. They claim the prism must be adding color to the light because white light has no colors in it. What is wrong with this claim?
AThe student is correct — prisms generate color through fluorescence
BWhite light is a mixture of all visible wavelengths; the prism spatially separates wavelengths already present by refracting each one by a different angle
CThe prism adds color, but only because of the glass's chemical composition reacting with photons
DThe student is correct that white light has no colors, but the spectrum comes from reflections inside the prism
White light contains all visible wavelengths simultaneously — it is a mixture, not a pure state. The prism exploits the wavelength-dependence of the refractive index (dispersion): shorter wavelengths (violet) experience a slightly higher n and bend more; longer wavelengths (red) bend less. The prism is a separator, not a generator. This is confirmed by the reverse experiment: focusing the dispersed spectrum back through a second prism recombines the colors into white light.
Question 2 Multiple Choice
Red light enters a glass prism. Compared to violet light entering the same prism at the same angle, red light exits with a smaller deflection angle. What is the direct physical reason?
ARed photons have higher energy and resist bending more than lower-energy violet photons
BRed light travels faster in vacuum than violet light, so it enters the glass at a larger angle
CRed light has a longer wavelength, experiences a lower refractive index in glass, and therefore bends less at each surface according to Snell's law
DThe prism absorbs red photons before they can be fully deflected, reducing their deflection
Snell's law states n₁ sin θ₁ = n₂ sin θ₂. A smaller n₂ (as glass has for red light) means a larger exit angle — less bending. Red light in typical glass has n ≈ 1.523 while violet has n ≈ 1.532, a small but sufficient difference to produce visible angular separation. The energy of the photon (red is lower energy than violet) does not directly enter Snell's law; it is the wavelength-dependence of n that drives dispersion. The refraction occurs twice — at entry and exit — and both refractions add to the angular spread.
Question 3 True / False
If white light passes through a glass slab with perfectly parallel faces (not a prism), the exiting beam is white, not a spectrum.
TTrue
FFalse
Answer: True
In a flat slab, both faces are parallel. Dispersion does occur at the first surface — each wavelength bends by a different angle. But at the second parallel surface, each wavelength bends back by exactly the same amount in the opposite direction, recombining all wavelengths to reconstruct the original direction and white appearance. A triangular prism has non-parallel surfaces, so the second refraction continues spreading the wavelengths apart rather than reversing the first. The geometry of the prism is essential; flat glass cannot produce a persistent spectrum.
Question 4 True / False
In a primary rainbow, red appears at the top (outside) of the arc and violet at the bottom (inside) — which is the opposite of the order colors exit a prism.
TTrue
FFalse
Answer: False
The order is not reversed relative to a prism. In both a prism and a rainbow, red bends the least and violet the most. In a primary rainbow, red appears at the outside (higher elevation angle, ~42° from the antisolar point) and violet at the inside (~40°). This matches the prism order: red bends least, exits at the shallowest angle relative to the incoming beam, and corresponds to droplets at the highest angular position above the antisolar point. The geometry differs from a prism (it involves total internal reflection inside spherical droplets), but the underlying dispersion — violet refracts more than red — is the same.
Question 5 Short Answer
Why does a triangular glass prism produce a visible spectrum when white light passes through it, while a rectangular glass slab with parallel faces does not?
Think about your answer, then reveal below.
Model answer: A prism's two refracting surfaces are angled relative to each other (non-parallel). At the first surface, each wavelength refracts by a slightly different amount (violet more, red less). At the second, non-parallel surface, refraction acts in the same angular direction, further increasing the angular spread. The divergence between red and violet accumulates across both surfaces. In a flat slab, the two parallel surfaces cause the second refraction to exactly cancel the first — each wavelength returns to its original direction and all colors recombine. The non-parallel geometry of a prism is the essential feature that prevents cancellation.
This is the geometric key to all dispersion devices. The triangular shape isn't cosmetic — it ensures the two surfaces cooperate in spreading wavelengths apart rather than working against each other. A direct consequence: two prisms arranged apex-to-base will reconstruct white light from a dispersed spectrum, confirming that the prism separates but doesn't destroy the original mixture.