A beam of light travels at different speeds through different substances due to their varying refractive indices. Among the substances you listed:
- Air has a refractive index very close to 1 (about 1.0003).
- Water has a refractive index of about 1.33.
- Glass has a refractive index that typically ranges from about 1.5 to 1.9, depending on the type of glass.
- Plastic generally has a refractive index ranging from about 1.4 to 1.7.
Since the refractive index of air is the lowest among these substances, light travels fastest in air.
Therefore, the answer is Air.
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