Refraction and Snell's Law

Light travels at different speeds in different materials. In a vacuum it moves at c, but in water or glass it slows down, because the index of refraction n of a material is defined as c divided by the light's speed in that material. Whenever a ray crosses the boundary between two media at an angle, one edge of the wavefront reaches the new medium and slows down before the rest of it does. That mismatch in speed across the wavefront makes it swing around, like a car veering when one set of wheels hits mud while the other is still on pavation. The result is a change in direction: the ray bends toward the normal when entering a slower (higher n) medium, and away from the normal when entering a faster (lower n) medium.

Snell's law makes this precise: n1 sin(theta1) = n2 sin(theta2), where theta1 and theta2 are angles measured from the normal, the line perpendicular to the surface at the point where the ray hits. Given any three of the four quantities, you can solve for the fourth. A straw in water looks snapped because light leaving the submerged part bends as it exits into air, shifting where your eye thinks the straw is.

This sits right after reflection, where angle in equals angle out with no speed change at all, and right before total internal reflection and lenses, where the same bending rule explains why rays converge to form images. Tracing a ray through a rectangular block uses Snell's law twice, once entering and once leaving, and the two bends cancel so the exit ray is parallel to the original, just shifted sideways.