Rami Arieli: "The Laser Adventure" Chapter 1.1.2 page 2

Wavelength in Matter:
We saw that the velocity of light in matter is slower than in vacuum. This slower velocity is associated with reduced wavelength: l = l₀/n , while the frequency remains the same (see figure 1.5). Figure 1.5: Change of wavelength in matter

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An interactive simulation can help you see how a wave changes when crossing the boundary between different media. Click on Wave-Change Applet 

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