Yes, wavelength changes when a wave moves from one medium to another. The speed of the wave changes too, but the frequency stays the same. Since speed equals frequency times wavelength (v = fλ), and frequency is locked in by the original source, wavelength has to adjust whenever speed changes.
Why Frequency Stays the Same
When a wave hits a boundary between two materials, the particles on one side of the boundary are driving the particles on the other side. Those particles in the new medium vibrate at exactly the same rate as the ones pushing them. There’s no mechanism for the frequency to change at a boundary, so it passes through unchanged.
This is true for all types of waves: light, sound, water waves, waves on a string. The frequency is set by the source and stays constant no matter how many different materials the wave passes through.
How Wavelength Adjusts
The fundamental wave equation ties everything together: v = fλ. The speed of a wave depends on the properties of the medium it’s traveling through, not on the source. Frequency depends entirely on the source. That leaves wavelength as the variable that absorbs the difference. If the wave slows down in the new medium, the wavelength gets shorter. If it speeds up, the wavelength stretches out.
Think of it like cars on a highway merging into a construction zone. The cars keep arriving at the same rate (frequency), but they’re forced to slow down (speed drops), so they bunch closer together (shorter wavelength). Once they leave the construction zone and speed up again, the spacing stretches back to normal.
Light: A Concrete Example
Light in a vacuum travels at exactly 299,792,458 meters per second. When it enters a transparent material like water or glass, it slows down. The ratio of the vacuum speed to the speed in the material is called the refractive index (n). Water has a refractive index of 1.333, crown glass is 1.517, and diamond is 2.417, all measured for yellow light at 589 nanometers.
The wavelength inside a material follows a simple formula: divide the vacuum wavelength by the refractive index. So that 589-nanometer yellow light shrinks to about 442 nm in water (589 ÷ 1.333) and roughly 244 nm in diamond (589 ÷ 2.417). The higher the refractive index, the more the wavelength compresses.
Once the light exits the material and returns to air or vacuum, the wavelength returns to its original value. The change is only present while the wave is inside the medium.
What About Sound?
Sound behaves the same way, just with different numbers. Sound travels faster in denser, stiffer materials, which is the opposite pattern from light. In air at room temperature, sound moves at about 343 meters per second. In water, it jumps to roughly 1,480 m/s. Since the frequency stays constant, the wavelength of a sound wave in water is about four times longer than it is in air for the same tone.
Why This Matters: Dispersion and Rainbows
The refractive index of a material isn’t perfectly constant. It varies slightly depending on the wavelength of the light passing through. Violet light, which has a shorter wavelength, is always refracted more strongly than red light in the visible range. This means different colors slow down by different amounts inside glass or water, and their wavelengths compress by different amounts.
When white light enters a glass prism, each color bends at a slightly different angle because of this wavelength-dependent speed change. The colors fan out into the familiar rainbow spectrum. The same principle creates rainbows in the sky, where water droplets act as tiny prisms. It also causes chromatic aberration in camera lenses, where different colors focus at slightly different points because the lens bends them unequally.
Quick Reference
- Frequency: set by the source, unchanged by the medium
- Speed: determined by the medium’s physical properties
- Wavelength: adjusts to satisfy v = fλ, so it changes whenever speed changes
- Direction: if the wave enters at an angle, it also bends (refraction), following Snell’s law: n₁ sin θ₁ = n₂ sin θ₂
The wavelength change is real and measurable while the wave is inside the medium, but it’s completely reversible. As soon as the wave exits back into its original medium, the original wavelength is restored.