The Memory Trick
💡 Violet Bends Most, Red Bends Least
Dispersion is the splitting of white light into its full spectrum of colors when passing through a material like a glass prism. This happens because a material's refractive index isn't quite identical for every wavelength — it varies slightly across the visible spectrum. Violet light, with the highest refractive index, bends (refracts) the most; red light, with the lowest refractive index, bends the least — spreading the originally combined white light into a visible rainbow.
Why It Works
Since Snell's Law directly ties the amount of bending to the refractive index, and different colors of light experience very slightly different refractive indices in the same material, each color bends by a very slightly different amount when passing through the same prism — over the full visible spectrum, that difference is enough to visibly separate the colors into a spread-out rainbow pattern.
Step by Step
Understanding Dispersion
1
A prism deliberately exploits this effect
A triangular glass prism refracts light twice (once entering, once exiting), doubling the color-separating effect of dispersion and producing a clearly visible spread-out spectrum.
Isaac Newton's classic prism experiments first demonstrated conclusively that white light is actually composed of all the visible colors combined, using exactly this dispersion effect.
2
Rainbows use the same principle, plus internal reflection
Natural rainbows form when sunlight enters spherical water droplets in the atmosphere, undergoes dispersion (splitting into colors) upon refraction, reflects once off the inside back surface of the droplet, and refracts again exiting — the combination of dispersion and internal reflection produces the visible rainbow arc.
This is why rainbows always appear in a consistent color order (red on the outer edge, violet on the inner edge) — a direct consequence of violet bending more sharply than red at each refraction.
3
Dispersion is generally an unwanted effect in lens design
While beautiful and useful in prisms, dispersion is actually a problem (chromatic aberration) in camera and telescope lenses, where you generally want all colors to focus at exactly the same point — lens designers use special combinations of glass types to minimize this unwanted dispersion effect.
High-quality camera lenses often use 'low-dispersion' glass elements specifically engineered to minimize the color-separating effect that would otherwise blur fine image detail.
🏥 Worked Example
White light passes through a glass prism. Explain why violet light exits the prism bent at a noticeably different angle than red light, even though both entered the prism together as part of the same white light beam.
1
Recall that refractive index varies slightly by wavelength: glass has a slightly higher refractive index for violet light than for red light.
2
Apply Snell's Law to each color separately: since violet light experiences a higher n in the glass, it bends more sharply (toward the normal) than red light does at each refraction.
3
Conclusion: despite entering together as combined white light, violet and red components refract by measurably different amounts inside (and exiting) the prism, causing them to visibly separate into distinct colors by the time they exit — this is dispersion.
📌 Exam Application
Exams test explaining WHY dispersion occurs (wavelength-dependent refractive index), correctly identifying that violet bends most and red bends least, and connecting dispersion to both prisms and natural rainbow formation.
⚠️ Most Common Dispersion of Light Mistakes
The most common trap is thinking dispersion happens because different colors have different SPEEDS in a vacuum (they don't — all colors of light travel at exactly c in vacuum) — dispersion specifically arises from different colors having slightly different refractive indices (and therefore different speeds) WITHIN a material like glass, not in vacuum.
✓ Quick Self-Test
1) What is dispersion? The splitting of white light into its component colors due to wavelength-dependent differences in refractive index. 2) Which color of visible light bends the most passing through a prism, and which bends the least? Violet bends most; red bends least. 3) Why does a prism separate white light into a visible spectrum? Different wavelengths (colors) have slightly different refractive indices in glass, causing each to refract by a slightly different amount. 4) What two optical effects combine to create a natural rainbow? Dispersion (splitting into colors) and internal reflection within water droplets. 5) Is dispersion generally a wanted or unwanted effect in camera and telescope lens design? Generally unwanted — it causes chromatic aberration, which lens designers try to minimize.