Why light bends: speed, medium and the normal

Why light bends: speed, medium and the normal

Light refracts because one side of a wavefront changes speed before the other when it enters a new medium at an angle. The part entering a slower medium first slows first, rotating the wavefront and changing the ray direction toward the normal. If light enters along the normal, both sides change speed together and the direction does not change, even though speed and wavelength do. Frequency is set by the source and remains the same across a boundary; speed and wavelength change together. This wavefront mechanism makes the ray rule more than a memorised drawing convention.

Work it through

A ray enters glass obliquely from air. In a wavefront sketch, the leading edge reaches glass first and slows, causing the whole front to pivot so the ray direction turns toward the normal. If the ray later leaves the glass into air, the leading edge speeds up first and the ray turns away from the normal. Use arrow direction to keep the sequence clear.

Mastery target

Explain refraction from a wavefront speed change, predict the bend relative to the normal and state which wave quantities change at a boundary.