It's not entirely accurate to say that a wave *undergoes diffraction* simply because it changes speed as it enters a new medium. While a change in speed can influence diffraction, it's not the sole cause.

Here's a breakdown:

Diffraction: This phenomenon occurs when a wave encounters an obstacle or an opening whose size is comparable to the wavelength of the wave. The wave then bends around the obstacle or spreads out through the opening.

Speed Change: When a wave transitions from one medium to another, its speed changes due to differences in the medium's properties (e.g., density, elasticity). This change in speed is governed by Snell's Law.

Relationship:

* Diffraction is affected by wavelength: A change in speed can alter the wavelength of the wave. Since diffraction depends on the wavelength, changes in speed can indirectly influence diffraction.

* Speed changes are not the sole cause of diffraction: Diffraction requires an obstacle or opening that's comparable to the wave's wavelength. A wave changing speed alone won't cause diffraction without an obstacle.

Example:

Imagine a light wave traveling from air to water. Light travels slower in water, so its speed changes. However, if the light wave continues in a straight line without encountering any obstacles or openings, it will not diffract, even though its speed has changed.

In summary:

* Speed changes can indirectly affect diffraction by changing the wavelength.

* Speed changes alone are not sufficient to cause diffraction. Diffraction requires an obstacle or opening.