Refraction results from a change of wave speed across a boundary; oblique entry can change the direction of travel. The frequency remains constant, so wavelength decreases if wave speed decreases, according to v = fλ. In the Higher-tier wavefront model, one part of a front reaches the slower region first and falls behind, causing the front to rotate. Entry along the normal changes speed and wavelength without changing direction. Use clear labels. Check your reasoning. Distinguish the quantities.