Dispersion of Light

Dispersion is the splitting of white light into its different colours when it passes through a prism. A triangular glass prism refracts each colour by a slightly different amount, so the colours spread out into a spectrum.

White Light and Monochromatic Light

White light is a mixture of many colours. Light that has only one colour (one frequency) is called monochromatic.

Why a Prism Splits Light

When light enters glass, it slows down and bends (refraction). The amount it slows down depends on colour. Violet light slows down more than red light in glass. This means violet bends more, and red bends less. In symbols: $n_{\text{violet}} > n_{\text{red}}$ and $v = \tfrac{c}{n}$, so a larger refractive index $n$ means a smaller speed $v$ in glass.

The Visible Spectrum

Traditional seven colours: Red, Orange, Yellow, Green, Blue, Indigo, Violet.

• Order of wavelength (longest to shortest): Red → Orange → Yellow → Green → Blue → Indigo → Violet
• Order of frequency (lowest to highest): Violet → Indigo → Blue → Green → Yellow → Orange → Red

Everyday Example: Rainbows

Raindrops act like tiny prisms. Sunlight enters a drop, refracts and disperses, reflects inside the drop, then refracts again as it leaves. The colours spread out to form a rainbow.

Common Misconceptions

• A prism does not add colour. It separates the colours already in white light.
• Violet bends the most; red bends the least in glass.
• Frequency does not change when light enters glass, but speed and wavelength do ($v = f\lambda$).