Dispersion of Light

Dispersion is the splitting of white light into its component colours when it passes through a prism (or any medium where speed varies with wavelength).

The seven colours produced are called the spectrum: VIBGYOR

Violet, Indigo, Blue, Green, Yellow, Orange, Red

When light enters a denser medium (like glass), different wavelengths (colours) slow down by different amounts:

• Violet has the shortest wavelength → slows down most → bends most (highest refractive index in glass)
• Red has the longest wavelength → slows down least → bends least (lowest refractive index in glass)

Refractive index in glass: Violet > Indigo > Blue > Green > Yellow > Orange > Red

So: Violet deviates most; Red deviates least.

1. White light enters the first face of a glass prism
2. Light refracts (bends) as it enters (denser medium)
3. Different wavelengths refract by different amounts → colours begin to separate
4. Light exits from the second face → second refraction further separates colours
5. Result: spectrum of colours emerges (VIBGYOR from bottom to top)

Note: A single flat glass slab does NOT produce dispersion because the second surface is parallel — the colours recombine.

1. Sunlight enters a spherical water droplet
2. Refracts at the front surface (white → separates slightly)
3. Total internal reflection at the back of the droplet
4. Refracts again at the front surface as it exits → full separation
5. Violet exits at ~40° from the incoming ray; Red at ~42°
6. Observer sees: Red on the outer arc, Violet on the inner arc

Why the rainbow is an arc: you only see a given colour from droplets at a specific angle relative to the sun and your eye — only a circle of droplets forms that angle.

If the spectrum from one prism is passed through an inverted second prism, the colours recombine into white light. This proves:

• Dispersion is a separation of components that were already in white light
• The prism does NOT add colours; it only separates them

Sky is blue: Short wavelengths (blue/violet) scattered by air molecules → appear to come from all directions → sky looks blue. Violet scattered even more, but our eyes are less sensitive to violet.

Sunset is red: When Sun is at horizon, light travels through more atmosphere → most blue scattered away → remaining light appears red/orange.

• VIBGYOR: Violet to Red — Violet deviates most, Red deviates least
• Higher refractive index for shorter wavelengths → more bending
• Rainbow: dispersion + total internal reflection inside water droplets
• Recombination proves white light is a mixture
• Sky colour (scattering) is a different phenomenon from dispersion

1. Why does a prism produce a spectrum but a glass slab does not?
2. Which colour has the highest speed inside a glass prism?
3. Why is the outer arc of a rainbow red and the inner arc violet?
4. What experiment proves that white light is a mixture of colours?
5. Stretch: If you look at a rainbow and then a second rainbow (double rainbow) behind it, what is different about the order of colours? Why?