The phenomenon whereby different wavelengths of light have different speeds in a medium is known as dispersion. Dispersion can be caused by a number of factors. 

1. **Geometry** The shape or structure of a medium can guide the waves of light, causing them to spread out.
2. **Physical Interactions** Light can physically interact with the atomic structure of a medium through electrons.
3. **Resonant Interactions** Wavelength is directly related to frequency, and light waves with frequencies near the resonant frequency of a material may be slowed more than light waves with differing frequencies.

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