Wavelength decreases in slower region due to constant frequency

Steps:

• Wave speed $v = f \lambda$, where $f$ is frequency (constant) and $\lambda$ is wavelength.
• In slower region, $v$ decreases, so $\lambda$ must decrease to keep $f$ constant.
• Waves appear compressed (shorter $\lambda$) in slow region.
• Returning to original speed, $\lambda$ increases back to original length.

Why A is correct:

• A depicts shorter wavelength in the middle (slow) region, matching $v = f \lambda$ with constant $f$.

Why the others are wrong:

• B shows longer wavelength in slow region, violating $v = f \lambda$.
• C shows wave bending like light refraction, but side view focuses on wavelength, not angle.
• D shows unchanged wavelength throughout, ignoring speed change.

Final answer: A