In 30 seconds
This short 3Blue1Brown explanation shows how different parts of a wavefront enter glass at different times, causing the wavefront and ray to turn.
- 1Shorter wavelengthThe frequency stays fixed while wavelength changes.
- 2A tilted boundaryOne side of a wavefront enters the slower medium first.
- 3A new directionThe ray turns as the wavefront changes angle.
Takeaways
- The frequency stays fixed while wavelength changes.
- One side of a wavefront enters the slower medium first.
- The ray turns as the wavefront changes angle.
Read the edited text by chapter
The wave slows
Speed and wavelength in glass.
When a light wave enters glass, its propagation speed in the material is lower than in a vacuum. Its frequency remains the same across a stationary boundary, so the wavelength in the glass is shorter.
To visualize the wave, imagine coloring the crests white and the spaces between them dark. The wavefronts become closer together inside the slower medium.
The wavefront turns
Why a ray changes direction at an angle.
If the boundary is tilted relative to an incoming wavefront, one side enters the glass and slows first. As the rest follows, the wavefront changes angle; the short clip shows this geometrically.
The ray direction is perpendicular to the wavefront in the uniform, isotropic materials considered here, so a change in wavefront angle means a change in propagation direction. The [full video](https://youtu.be/Cz4Q4QOuoo8) gives more context about refraction.
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