California Science Test (CAST) 8th Grade Practice Exam 2026 - Free 8th Grade Science Practice Questions and Study Guide

Diffraction explains why waves spread out after passing through a small opening. When a wave meets a narrow gap, the edges of the opening act like new sources of wavelets that radiate outward, causing the wave to bend and fan out on the other side. The tighter the opening relative to the wavelength, the more pronounced the spreading becomes, producing circular or curved wavefronts emanating from the gap. This behavior is observed with water waves, light passing through a slit, or sound moving through a small doorway.

In contrast, refraction involves bending at a boundary between two media with different speeds, not spreading from a gap; wavelength is a property of the wave, and energy describes how much wave is carried, not the way it spreads.